Low-temperature flow standard device flow range expansion method based on frequency conversion control

Abstract

The invention relates to the technical field of flow control and low-temperature engineering, in particular to a variable-frequency control-based flow range expansion method of a low-temperature flow standard device, which comprises the following steps of: acquiring basic working condition parameters and variable-frequency characteristic parameters of the low-temperature flow standard device through a parameter acquisition unit, and analyzing flow characteristics; a physical property parameter change rule and a flow characteristic comprehensive index are obtained, and a frequency conversion strategy is generated in combination with a frequency and flow characteristic curve; adjusting output frequency based on a frequency conversion strategy, and controlling pump rotating speed to perform flow expansion; the expansion result is verified, the expansion effect is judged, and after verification is finished, all verification results form a standardized document and are uploaded to a display terminal; the problem that continuous adjustment of the flow range from micro flow to medium and high flow is difficult is effectively solved, and the efficiency and stability of flow range expansion are improved.

Description

Technical Field

[0001] The present invention relates to the technical fields of flow control and cryogenic engineering, and specifically to a method for expanding the flow range of a cryogenic flow standard device based on variable frequency control. Background Technique

[0002] With the development of industries such as aerospace propulsion and liquefied natural gas, in the technical fields of flow control and cryogenic engineering, as the core equipment for calibrating cryogenic flow meters, the accuracy and stability of the flow range expansion of cryogenic flow standard devices directly affect the reliability of quantity transfer; with the introduction of variable frequency control technology, a technological leap from mechanical fixed frequency to electronic variable frequency has been achieved in this field. By adjusting the output frequency of the motor to change the pump speed, the flow regulation range of cryogenic flow standard devices initially meets the basic metering requirements for cryogenic media such as liquid hydrogen and liquid nitrogen. However, there are still many deficiencies in the existing technologies: Traditional technologies have not established a quantitative mapping model between cryogenic physical property parameters and flow characteristics. The variable frequency control strategy does not integrate multi-dimensional characteristic parameters, lacks a real-time monitoring and dynamic adjustment mechanism for temperature and pressure parameters, and the verification of the expansion effect does not consider the non-linear distribution of flow characteristics at low temperatures, resulting in difficulties in continuously adjusting the flow rate over a wide flow range from micro flow to medium and high flow rates, large flow measurement errors, and low expansion efficiency and stability.

[0003] To solve the above defects, a technical solution is provided now. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for expanding the flow range of a cryogenic flow standard device based on variable frequency control.

[0005] The purpose of the present invention can be achieved through the following technical solutions: A method for expanding the flow range of a cryogenic flow standard device based on variable frequency control includes: S1. Acquisition of device operation parameters: Obtain the basic operating conditions parameters and variable frequency characteristic parameters under different operating conditions of the cryogenic flow standard device through a parameter acquisition unit; S2. Flow characteristic analysis: Analyze the flow characteristics of the cryogenic flow standard device; S3. Generation of variable frequency strategy: Based on the results of the flow characteristic analysis, design a suitable variable frequency control strategy to effectively expand the flow range; S4. Execution of flow range expansion: Based on the determined frequency adjustment range of the variable frequency controller, control the pump speed by adjusting the output frequency of the variable frequency controller to effectively expand the flow range; S5. Verification of expansion result: Verify the flow range expansion result to judge the expansion effect.

[0006] As a further improvement of the present invention, the specific analysis process of S2 includes: Extract the working medium temperature, pressure, density, viscosity, and the upper and lower limits of the original flow range of the device from the basic operating condition parameters of the cryogenic flow standard device, and calculate the variation law of the physical property parameters of the working medium at different temperatures; Obtain the lowest working temperature and the highest working temperature of the cryogenic flow standard device, divide them based on the equal interval rule to generate an ordered temperature array, and obtain the temperature points; Based on the temperature designed under typical working conditions of the device as the reference temperature; obtain the fluid density parameter of the cryogenic flow standard device, and construct the mapping relationship between temperature and density ; obtain the density change rate of each temperature point based on the relative change rate ; By adopting a bypass sampling design, lead out a sampling branch pipe on the side of the main pipeline and deploy a viscosity sensor. The viscosity sensor excites the probe to vibrate through a piezoelectric ceramic, and measures the fluid viscosity by the vibration method to obtain the viscosity ; Obtain the viscosity change rate of each temperature point based on the relative change rate ; Obtain the flow points of the cryogenic flow standard device , where , is an integer; obtain the Reynolds number through the formula , where is the cross-sectional area of the probe vibration head; count all the Reynolds numbers to obtain the Reynolds number range . .

[0007] As a further improvement of the present invention, the specific analysis process of the flow characteristic analysis further includes: Obtain the nominal flow range from the device technical document or nameplate ; verify the effective flow range under the current working condition through actual testing , and obtain the effective index through the formula ; Analyze the obtained actual flow data, and obtain the flow stability index by calculating the ratio of the standard deviation of the flow data to the extreme difference of the flow data ; Extract the fluid density and viscosity of the cryogenic flow standard device in the low-temperature environment, and obtain the low-temperature influence index through the formula ; where is the dimensional change rate caused by the thermal expansion and contraction of the common materials of the flowmeter body and pipeline; is the density value at the reference temperature; is the viscosity at the reference temperature; are the influence weight factors of density, viscosity, and dimensional change rate respectively; Based on the output frequency of the variable frequency controller and the actual flow range, a frequency-flow characteristic curve is constructed by combining the least squares method ; where are the fitting coefficients, determined based on the measured data by the least squares method; is the frequency characteristic point; at the same time, through the formula the variable frequency sensitivity index is obtained , where represents the flow change amount; represents the frequency change amount; The effective index , the flow stability index , the low-temperature influence index , and the variable frequency sensitivity index After normalization processing, they are substituted into the formula to obtain the comprehensive flow characteristic index , where are the influence weight factors of the effective index, the flow stability index, the low-temperature influence index, and the variable frequency sensitivity index respectively.

[0008] As a further improvement of the present invention, the specific implementation process of the variable frequency strategy generation is as follows: Based on the frequency-flow characteristic curve, combined with the expected extended flow range , the frequency adjustment range of the variable frequency controller is determined ; Through the formula the minimum frequency is obtained; Through the formula the maximum frequency is obtained; where is the reference frequency, and are the frequency adjustment coefficients, obtained based on the variable frequency controller capacity, device physical limitations, and flow stability requirements; is the influence weight factor of the comprehensive flow characteristic index; The frequency adjustment step size is judged by the variable frequency sensitivity index, and the frequency adjustment step size is obtained by dividing the difference between the target flow and the initial flow by the variable frequency sensitivity index ; The frequency change rate is judged based on the flow stability index, and the actual adjustment rate is obtained through the formula ; where Adjust the rate based on...; is the influence weight factor of the flow stability index.

[0009] As a further improvement of the present invention, the specific implementation process of the flow range expansion execution includes: The variable frequency controller gradually increases or decreases the output frequency at a constant speed based on the acceleration or deceleration parameter of the actual adjustment rate; controls the speed of the AC motor by adjusting the output frequency of the pump power supply, and obtains the synchronous speed based on the synchronous speed formula of the asynchronous motor ; through the formula calculate the slip ratio , where is the actual speed; when the frequency is adjusted within the frequency adjustment range, the slip ratio changes little. At the same time, the pump is directly connected to the motor through a coupling, and the pump speed is the same as the motor speed. Therefore, the pump speed is directly determined by the input frequency of the motor, and a linear proportional relationship between the pump speed and the frequency is constructed; During the process of increasing or decreasing the output frequency, ensure that the parameters are within the allowable range during the adjustment process based on the frequency adjustment step size, and collect data at fixed time intervals through the temperature sensor and the pressure sensor to monitor the temperature parameters and pressure parameters of the fluid in the device in real time, and compare and judge with the allowable temperature range and pressure range of the normal operation of the device; When the temperature parameter or the pressure parameter is not within the allowable pressure range, it is determined that the parameter is abnormal, and then immediately adjust the actual adjustment rate of the output frequency. If the sampled data exceeds the parameter range for 3 consecutive times, then suspend the output frequency adjustment, keep the current frequency unchanged, and gradually reduce the output frequency at a decreasing speed of 0.5Hz / s until the temperature parameter and the pressure parameter are continuously collected 5 times and restored to the normal range, and then gradually restore the frequency adjustment at a rate of 30% of the original deceleration.

[0010] As a further improvement of the present invention, the specific implementation process of the flow range expansion execution also includes: During the frequency adjustment and flow expansion process, implement stability control measures, including using the PID control algorithm to maintain flow stability, and the control parameters are dynamically adjusted based on the change range of the Reynolds number; when the Reynolds number is less than the preset threshold, reduce the proportional coefficient to avoid overshoot; when the Reynolds number is greater than the preset threshold, enhance the differential action to suppress turbulent fluctuations; detect and suppress the flow fluctuations caused by external disturbances based on the disturbance suppression technology; achieve smooth transition between different frequency points based on the curve switching strategy to avoid flow mutation.

[0011] As a further improvement of the present invention, the specific verification steps of the expansion result verification are as follows: The expected expanded flow range , based on the logarithmic law, evenly select 7 test points; By using a Coriolis mass flowmeter to conduct triple repeated measurements of more than 120 seconds at each test point, and taking the average value as the actual flow value after removing transient data based on the Pauta criterion ; Through the formula Obtain the error index of the test point , where Is the allowable absolute error; Is the low-temperature compensation factor; Is the target flow value; When the error index of all test points meets the range less than the preset threshold, it is determined that the expansion is successful, and the relevant parameters of the current variable-frequency controller are locked, including the frequency adjustment range, acceleration, and deceleration; When there is When the error index of the test point exceeds the preset threshold range, it is determined as local correction, and the test point with the error is automatically located and the PID parameter iterative optimization is executed; When there are more than 3 test points with error indices greater than the preset threshold range, restore the original settings and record the failure information to generate a log; after the verification is completed, form all verification results into a standardized document and synchronously upload it to the display terminal.

[0012] Compared with the prior art, the beneficial effects of the present invention are: By constructing the mapping relationship between temperature and density and viscosity, combining the analysis of the Reynolds number change range, quantifying the influence of the low-temperature environment on fluid characteristics, generating a comprehensive flow characteristic index, generating a frequency adjustment range based on the comprehensive flow characteristic index, and optimizing the adjustment step and rate in combination with the variable-frequency sensitivity index and flow stability index, the present invention effectively solves the problem of large measurement errors caused by fluctuations in physical property parameters in the traditional technology, and significantly improves the efficiency and stability of flow range expansion. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] For the convenience of those skilled in the art to understand, the present invention will be further described below with reference to the accompanying drawings; Figure 1 Is the method flowchart of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0015] It should be understood that the terms "comprising" and "including" as used in the specification and claims of this disclosure indicate the presence of the described features, wholes, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, wholes, steps, operations, elements, components, and / or their combinations.

[0016] It should also be understood that the terms used in this disclosure specification are for the purpose of describing particular embodiments only and are not intended to limit this disclosure. As used in this disclosure specification and claims, unless the context clearly indicates otherwise, the singular forms "a", "an", and "the" are intended to include the plural forms. It should further be understood that the term "and / or" as used in this disclosure specification and claims refers to any combination and all possible combinations of one or more of the associated listed items and includes these combinations.

[0017] Please refer to Figure 1 As shown, the present invention is a method for expanding the flow range of a cryogenic flow standard device based on variable frequency control, including: obtaining device operation parameters, analyzing flow characteristics, generating a variable frequency strategy, executing the flow range expansion, and verifying the expansion effect.

[0018] S1. Obtaining device operation parameters: Obtain the basic operating condition parameters of the cryogenic flow standard device through a parameter acquisition unit. Among them, the basic operating condition parameters of the cryogenic flow standard device include: the working medium temperature C, pressure B, density , viscosity and the upper and lower limit values of the original flow range of the device. At the same time, obtain the variable frequency characteristic parameters of the flow standard device under different operating conditions through the parameter acquisition unit; S2. Analyzing flow characteristics: Analyze the flow characteristics of the cryogenic flow standard device, and the specific analysis method is as follows: 201. Extract the working medium temperature, pressure, density, viscosity, and the upper and lower limit values of the original flow range of the device from the basic operating condition parameters of the cryogenic flow standard device; and calculate the variation law of the physical property parameters of the working medium at different temperatures. The variation law of the physical property parameters includes the density change rate, viscosity change rate, and Reynolds number change range; 201-1. Density change rate: Obtain the lowest working temperature and the highest working temperature of the cryogenic flow standard device from the parameter acquisition unit, divide them based on the equal interval law of and generate an ordered temperature array ; calculate the temperature point through the formula , where , where n is an integer; the temperature designed under typical operating conditions of the device is used as the reference temperature , meanwhile, obtain the fluid density parameter of the low-temperature flow standard device, and construct the mapping relationship between temperature and density ; Through the formula calculate the density change rate at each temperature point ; Among them, is the density value at the temperature point; is the density value at the reference temperature; 201-2. Viscosity change rate: By adopting a bypass sampling design, a sampling branch pipe is led out from the side of the main pipeline, and the viscosity sensor is deployed on the sampling branch pipe to avoid interference with the measurement caused by too high a flow rate in the main pipeline; The viscosity sensor excites the probe to vibrate through piezoelectric ceramics. The change in fluid viscosity will change the vibration frequency of the probe. Through the formula calculate the viscosity , where is the fluid density; is the immersion depth of the probe vibration head; is the cross-sectional area of the probe vibration head; is the no-load frequency; is the vibration frequency in the fluid; Through the formula calculate the viscosity change rate at each temperature point ; Among them, is the viscosity at the temperature point; is the viscosity at the reference temperature; 201-3. Reynolds number change range: Obtain the flow rate points of the low-temperature flow standard device from the parameter acquisition unit , where , is an integer; Through the formula calculate the Reynolds number , and count the Reynolds numbers under all operating conditions to obtain the Reynolds number range ; 202. Comprehensive analysis of flow characteristics: 202-1. Obtain the nominal flow rate range from the device technical document or nameplate ; Verify the effective flow rate range under the current operating conditions through actual tests , and calculate the effective index through the formula , indicating the deviation degree between the basic performance of the device and the design specifications; 202-2. Conduct flow stability analysis on the obtained actual flow rate data, and calculate the flow stability index through the formula , where is the flow value of the th sampling point, is the average flow rate, is the number of sampling points; the smaller the fluctuation degree of the flow stability index, the more stable the flow; 202-3. Extract the fluid density and viscosity of the low-temperature flow standard device in the low-temperature environment through the parameter acquisition unit, and calculate the low-temperature influence index through the formula ; among them, is the dimensional change rate caused by the thermal expansion and contraction of the common materials of the flowmeter body and pipeline; are the influence weight factors of density, viscosity and dimensional change rate respectively; 202-4. Based on the output frequency of the frequency conversion controller and the actual flow range, construct the frequency-flow characteristic curve ; among them, is the fitting coefficient, which is determined based on the measured data through the least squares method; is the frequency characteristic point; at the same time, calculate the frequency conversion sensitivity index through the formula , where represents the flow change amount; represents the frequency change amount; 202-5. Normalize the effective index , the flow stability index , the low-temperature influence index and the frequency conversion sensitivity index , and substitute them into the formula to calculate the comprehensive flow characteristic index , where are the influence weight factors of the effective index, the flow stability index, the low-temperature influence index and the frequency conversion sensitivity index respectively; S3. Frequency conversion strategy generation: Based on the flow characteristic analysis results, design a suitable frequency conversion control strategy to effectively expand the flow range; the specific generation method is as follows: 301. Determine the expansion target. Based on the frequency-flow characteristic curve and combined with the expected expanded flow range , determine the frequency adjustment range of the frequency conversion controller; Calculate the minimum frequency through the formula ; Calculate the maximum frequency through the formula ; Among them, is the reference frequency, and is the frequency adjustment coefficient, which is obtained based on the capacity of the variable frequency controller, the physical limitations of the device, and the requirements for flow stability; is the influence weight factor of the comprehensive index of flow characteristics; 302. Design of variable frequency control parameters: Frequency adjustment step size: The frequency adjustment step size is judged based on the variable frequency sensitivity index, and the frequency adjustment step size is calculated through the formula where, is the initial flow rate, is the target flow rate; Frequency adjustment rate: The frequency change rate is judged based on the flow stability index, and the actual adjustment rate is calculated through the formula where, is the basic adjustment rate; is the influence weight factor of the flow stability index; S4. Execution of flow range expansion: Based on the determined frequency adjustment range of the variable frequency controller, the output frequency of the variable frequency controller is adjusted to control the pump speed, so as to effectively expand the flow range; The specific implementation process includes: 401. Before executing the expansion operation, the low-temperature flow standard device performs the following preparatory work: Equipment inspection: Confirm that the flow standard device and the variable frequency control system are working properly; Parameter setting: Load the generated variable frequency strategy parameters into the variable frequency controller; Environmental stability: Ensure that the low-temperature environmental conditions are stable and the temperature fluctuation is within the allowable range; Device preheating: Based on the standard flow rate, the device is preheated to make each component reach a stable working state, and the reference flow rate value at the reference frequency is recorded; 402. Execution of frequency adjustment: The variable frequency controller gradually increases or decreases the output frequency at a constant speed based on the acceleration or deceleration parameters of the actual adjustment rate; The variable frequency controller controls the speed of the AC motor by adjusting the output frequency of the pump power supply, and the synchronous speed is calculated based on the synchronous speed formula of the asynchronous motor , and the slip ratio is calculated through the formula where, is the actual speed; In variable frequency control, when the frequency is adjusted within the frequency adjustment range, the slip ratio changes little. At the same time, the pump is directly connected to the motor through a coupling, and the pump speed is the same as the motor speed. Therefore, the pump speed is directly determined by the input frequency of the motor, and a linear proportional relationship between the pump speed and the frequency is constructed where, is the pump speed, is the output frequency of the variable frequency controller, and is a preset proportionality coefficient; as the output frequency increases or decreases, the pump speed rises or falls accordingly; During the process of decreasing the output frequency, based on the frequency adjustment step size, ensure that the parameters are within the allowable range during the adjustment process, and continuously monitor the temperature parameter and pressure parameter of the fluid in the device; through the temperature sensor and pressure sensor, collect data at fixed time intervals, and compare the collected temperature parameter and pressure parameter with the allowable temperature range and pressure range of the normal operation of the device for comparison and judgment; When the temperature parameter is not within the allowable range or the pressure parameter is not within the allowable pressure range, it is determined that the parameter is abnormal, and then immediately adjust the actual adjustment rate of the output frequency. If the sampled data exceeds the parameter range for 3 consecutive times, then suspend the output frequency adjustment, keep the current frequency unchanged, and gradually reduce the output frequency at a decreasing rate of 0.5 Hz / s until the temperature parameter and pressure parameter are continuously sampled 5 times and restored to the normal range, and then gradually restore the frequency adjustment at a rate of 30% of the original deceleration; 403. Stability control: During the frequency adjustment and flow rate expansion process, implement stability control measures. The specific control measures include: Maintain the flow rate stability by adopting the PID control algorithm, and the control parameters are dynamically adjusted based on the change range of the Reynolds number; when the Reynolds number is less than the preset threshold, the fluid inertia is small, so reduce the proportionality coefficient to avoid overshoot; when the Reynolds number is greater than the preset threshold, enhance the differential action to suppress the turbulent fluctuation; Detect and suppress the flow rate fluctuation caused by external interference based on the disturbance suppression technology; achieve smooth transition between different frequency points based on the curve switching strategy to avoid sudden changes in the flow rate; S5. Expansion result verification: Verify the expansion effect by verifying the flow rate range expansion result; the specific verification steps are as follows: For the expected expanded flow rate range , evenly select 7 test points based on the logarithmic law; calculate the test points through the formula , where , respectively represent the lower limit of the expanded flow rate, the median flow rate, and the upper limit of the flow rate, and other test points evenly cover the expanded flow rate interval; Use a Coriolis mass flowmeter to perform triple retesting on each test point, with each measurement duration greater than 120 seconds, and take the average value as the actual flow rate value after removing transient data based on the Pauta criterion ; Calculate the test point error index through the formula ​ , where is the allowable absolute error; is the low-temperature compensation factor; is the target flow value; When all test points meet the error index less than the preset threshold range, it is determined that the expansion is successful, and the relevant parameters of the current variable-frequency controller are locked. The relevant parameters include the frequency adjustment range, acceleration, and deceleration; When there is where the error index of the test point is greater than the preset threshold range, it is determined as local correction, and the test point with the error is automatically located and the PID parameter iterative optimization is executed; When there are more than 3 test points where the error index is greater than the preset threshold range, it is determined that the expansion fails, and the abnormal fallback function is activated. By reading and restoring the original frequency settings of the device before the flow range expansion, the device is restored to the initial stable operation state. At the same time, the detailed information of the expansion failure is recorded. The detailed information includes the failed test points, error values, and relevant parameters of the variable-frequency controller; an error log is generated and uploaded to the display terminal, and technicians can judge the reason for the expansion failure based on the error log and troubleshoot and improve the relevant parameters of the variable-frequency controller or the device hardware; After the verification is completed, all verification results are formed into a standardized document. The document content includes a comparison table of the flow ranges before and after the expansion, the measurement data and statistical analysis results of each test point, an accuracy evaluation report of the expanded area, a curve of the relationship between variable-frequency parameters and flow, recommended operating parameters and precautions, and a long-term stability monitoring plan, and is uploaded to the display terminal synchronously.

[0019] The preferred embodiments of the present invention disclosed above are only used to help explain the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the present invention to only the specific implementation manners. Obviously, many modifications and variations can be made according to the content of this specification. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of the present invention, so that those skilled in the art in the relevant technical field can understand and utilize the present invention well. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A method for expanding the flow range of a low-temperature flow standard device based on variable frequency control, characterized in that, Including: S1. Obtaining device operation parameters: Obtain the basic operating conditions parameters and the frequency conversion characteristic parameters under different operating conditions of the cryogenic flow standard device through the parameter acquisition unit; S2. Flow characteristic analysis: Analyze the flow characteristics of the cryogenic flow standard device; S3. Generating frequency conversion strategy: Based on the results of the flow characteristic analysis, design a suitable frequency conversion control strategy to effectively expand the flow range; S4. Executing flow range expansion: Based on the determined frequency adjustment range of the frequency conversion controller, control the pump speed by adjusting the output frequency of the frequency conversion controller to effectively expand the flow range; S5. Verifying expansion results: Verify the flow range expansion results to judge the expansion effect.

2. The method for expanding the flow range of the low-temperature flow standard device based on variable frequency control according to claim 1, wherein The specific analysis process of S2 includes: Extract the working medium temperature, pressure, density, viscosity, and the upper and lower limits of the original flow range of the cryogenic flow standard device from the basic operating conditions parameters to calculate the variation law of the physical property parameters of the working medium at different temperatures; Obtain the lowest working temperature and the highest working temperature of the cryogenic flow standard device, divide them based on the equal interval rule to generate an ordered temperature array, and obtain temperature points; The temperature designed based on the device under typical working conditions is used as the reference temperature; obtain the fluid density parameters of the low-temperature flow standard device and construct the mapping relationship between temperature and density ; Obtain the density change rate at each temperature point based on the relative change rate ; By adopting a bypass sampling design, a sampling branch pipe is led out from the side of the main pipeline and a viscosity sensor is deployed. The viscosity sensor excites the probe rod to vibrate through a piezoelectric ceramic, and the viscosity of the fluid is measured by the vibration method to obtain the viscosity ; Obtain the viscosity change rate at each temperature point based on the relative change rate ; Obtain the flow points of the low-temperature flow standard device , where , is an integer; the Reynolds number is obtained through the formula , where is the cross-sectional area of the probe vibration head; the Reynolds number range is obtained by counting all the Reynolds numbers .​ 3. The flow range expansion method of the cryogenic flow standard device based on variable frequency control according to claim 2, wherein The specific analysis process of the flow characteristic analysis further includes: Obtain the nominal flow rate range from the device technical documentation or nameplate ; Verify the effective flow rate range under the current working conditions through actual tests , through the formula to obtain the effective index ; Analyze the obtained actual flow data, and obtain the flow stability index by calculating the ratio of the standard deviation of the flow data to the extreme difference of the flow data ; Extract the fluid density and viscosity of the low-temperature flow standard device in a low-temperature environment and viscosity , and obtain the low-temperature influence index through the formula ; where ; among them, is the dimensional change rate caused by the thermal expansion and contraction of the common materials of the flowmeter body and pipeline; is the density value at the reference temperature; is the viscosity at the reference temperature; are the influence weight factors of density, viscosity, and dimensional change rate respectively; Based on the output frequency of the variable frequency controller and the actual flow range, a frequency-flow characteristic curve is constructed by combining the least squares method ; where is the fitting coefficient, which is determined by the least squares method based on the measured data; is the frequency characteristic point; meanwhile, the variable frequency sensitivity index is obtained through the formula , where represents the flow change; represents the frequency change;​ The effective exponent , the flow stability index , the low-temperature influence index and the frequency conversion sensitivity index are normalized and then substituted into the formula to obtain the comprehensive flow characteristic index , where are the influence weight factors of the effective exponent, the flow stability index, the low-temperature influence index and the frequency conversion sensitivity index respectively.

4. The flow range expansion method of the cryogenic flow standard device based on variable frequency control according to claim 1, characterized in that, The specific implementation process of the generating frequency conversion strategy is as follows: Based on the frequency and flow characteristic curves and combined with the expected expanded flow range , determine the frequency adjustment range of the variable frequency controller ; Obtained by the formula the minimum frequency is obtained ; Obtained by the formula to obtain the maximum frequency ; Among them, is the reference frequency, and are frequency adjustment coefficients, obtained based on the capabilities of the variable frequency controller, the physical limitations of the device, and the requirements for flow stability; is the influence weight factor of the comprehensive index of flow characteristics; The frequency adjustment step size is determined by the frequency conversion sensitivity index. The frequency adjustment step size is obtained by dividing the difference between the target flow rate and the initial flow rate by the frequency conversion sensitivity index. ; Judging the frequency change rate based on the flow stability index, and obtaining the actual adjustment rate through the formula ; where is the basic adjustment rate; is the influence weight factor of the flow stability index.

5. The method for expanding the flow range of the low-temperature flow standard device based on variable frequency control according to claim 1, wherein The specific implementation process of the executing flow range expansion includes: The variable frequency controller gradually increases or decreases the output frequency at a constant speed based on the acceleration or deceleration parameter of the actual adjustment rate; the rotation speed of the AC motor is controlled by adjusting the output frequency of the pump power supply, and the synchronous speed is obtained based on the synchronous speed formula of the asynchronous motor. ; Through the formula Calculate the slip rate , where is the actual rotation speed; when the frequency is adjusted within the frequency adjustment range, the slip rate changes little. At the same time, the pump is directly connected to the motor through a coupling, and the pump rotation speed is the same as the motor rotation speed. Therefore, the pump rotation speed is directly determined by the motor input frequency, and a linear proportional relationship between the pump rotation speed and the frequency is constructed. During the process of increasing or decreasing the output frequency, ensure that the parameters are within the allowable range during the adjustment process based on the frequency adjustment step size, and collect data at fixed time intervals through the temperature sensor and the pressure sensor to real-time monitor the temperature parameters and pressure parameters of the fluid in the device, and compare and judge with the allowable temperature range and pressure range of the normal operation of the device; When the temperature parameter or the pressure parameter is not within the allowable pressure range, it is determined that the parameter is abnormal, and then immediately adjust the actual adjustment rate of the output frequency. If the sampled data exceeds the parameter range for 3 consecutive times, then suspend the output frequency adjustment, keep the current frequency unchanged, and gradually reduce the output frequency at a decreasing rate of 0.5Hz / s until the temperature parameter and the pressure parameter are continuously sampled and restored to the normal range for 5 times, and then gradually restore the frequency adjustment at a rate of 30% of the original deceleration rate.

6. The method for expanding the flow range of the cryogenic flow standard device based on variable frequency control according to claim 5, characterized in that, The specific implementation process of the executing flow range expansion further includes: During the frequency adjustment and flow expansion process, implement stability control measures, including using the PID control algorithm to maintain flow stability, and the control parameters are dynamically adjusted based on the change range of the Reynolds number; when the Reynolds number is less than the preset threshold, reduce the proportional coefficient to avoid overshoot; when the Reynolds number is greater than the preset threshold, enhance the differential action to suppress the turbulent fluctuation; detect and suppress the flow fluctuation caused by external interference based on the disturbance suppression technology; achieve smooth transition between different frequency points based on the curve switching strategy to avoid flow mutation.

7. The method for expanding the flow range of the low-temperature flow standard device based on variable frequency control according to claim 1, characterized in that The specific verification steps of the verifying expansion results are as follows: The expected extended flow range , and 7 test points are evenly selected based on the logarithmic law; At each test point, triple repeated measurements are taken for more than 120 seconds using a Coriolis mass flowmeter. After removing transient data based on the Chauvenet's criterion, the average value is taken as the actual flow rate value. ; Through the formula the test point error index is obtained , where is the allowable absolute error; is the low-temperature compensation factor; is the target flow value; When all test points meet the error index less than the preset threshold range, it is determined that the expansion is successful, and lock the relevant parameters of the current frequency conversion controller, including the frequency adjustment range, acceleration, and deceleration; When there is a test point error index greater than the preset threshold range at a certain point, it is determined as local correction, and automatically locates to the test point with errors and performs PID parameter iterative optimization; When there are more than 3 test point error indicators greater than the preset threshold range, restore the original settings, record the failure information, and generate a log; after the verification is completed, form all verification results into a standardized document and synchronously upload it to the display terminal.

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