Flow range expansion method of low-temperature flow standard device based on variable frequency control

Abstract

The present invention relates to the field of flow control and cryogenic engineering technology, and specifically to a flow range expansion method for a cryogenic flow standard device based on variable frequency control. The method comprises the following steps: a parameter acquisition unit is used to acquire basic operating parameters and variable frequency characteristic parameters of the cryogenic flow standard device, and the flow characteristics are analyzed to obtain a variation law of physical parameters and a comprehensive index of flow characteristics. A variable frequency strategy is generated by combining frequency and flow characteristic curves; the output frequency is adjusted based on the variable frequency strategy, and the pump speed is controlled to expand the flow; the expansion result is verified, and the expansion effect is judged. After the verification is completed, all verification results are formed into a standardized document and uploaded to a display terminal. The method effectively solves the problem of difficulty in continuously adjusting the flow range from micro flow to medium and high flow, and improves the efficiency and stability of the flow range expansion.

Description

Technical Field

[0001] The present invention relates to the field of flow control and cryogenic engineering technology, and in particular to a method for expanding the flow range of a cryogenic flow standard device based on variable frequency control. Background Art

[0002] With the development of industries such as aerospace propulsion and liquefied natural gas, cryogenic flow standards are becoming increasingly important in the fields of flow control and cryogenic engineering. As core equipment for calibrating cryogenic flowmeters, their flow range expansion, accuracy, and stability directly impact the reliability of value transfer. With the introduction of variable frequency control technology, this field has achieved a technological leap from mechanical fixed frequency to electronic variable frequency. By adjusting the motor output frequency to change the pump speed, the flow regulation range of cryogenic flow standards has initially met the basic measurement requirements of cryogenic media such as liquid hydrogen and liquid nitrogen. However, existing technologies still have many shortcomings:

[0003] Traditional technologies have not established a quantitative mapping model between low-temperature physical properties and flow characteristics, the variable frequency control strategy has not integrated multi-dimensional characteristic parameters, and there is a lack of real-time monitoring and dynamic adjustment mechanisms for temperature and pressure parameters. The expansion effect verification has not considered the nonlinear distribution of flow characteristics at low temperatures, resulting in difficulties in continuous adjustment covering a wide flow range from micro-flow to medium and high flow, large flow measurement errors, and low expansion efficiency and stability.

[0004] In order to solve the above-mentioned defects, a technical solution is now provided. Summary of the Invention

[0005] The object of the present invention is to provide a method for expanding the flow range of a low-temperature flow standard device based on variable frequency control.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] The flow range expansion method of a low-temperature flow standard device based on variable frequency control includes:

[0008] S1. Acquisition of device operating parameters: Acquisition of basic operating parameters of the low-temperature flow standard device and frequency conversion characteristic parameters under different operating conditions through the parameter acquisition unit;

[0009] S2. Flow characteristics analysis: Analyze the flow characteristics of the low-temperature flow standard device;

[0010] S3. Frequency conversion strategy generation: Based on the flow characteristics analysis results, design a suitable frequency conversion control strategy to achieve effective expansion of the flow range;

[0011] S4, flow range expansion execution: Based on the determined frequency adjustment range of the frequency converter, the output frequency of the frequency converter is adjusted to control the pump speed, thereby achieving effective expansion of the flow range;

[0012] S5. Verification of expansion results: Determine the expansion effect by verifying the flow range expansion results.

[0013] As a further improvement of the present invention, the specific analysis process of S2 includes:

[0014] The working medium temperature, pressure, density, viscosity and the upper and lower limits of the original flow range of the device are extracted from the basic operating parameters of the low-temperature flow standard device to calculate the change law of the physical properties of the working medium at different temperatures;

[0015] Obtain the lowest and highest operating temperatures of the low-temperature flow standard device, divide it based on the equal interval rule, generate an ordered array of temperatures, and obtain the temperature points;

[0016] Based on the design temperature of the device under typical working conditions as the reference temperature; obtain the fluid density parameters of the low-temperature flow standard device and construct a mapping relationship between temperature and density ; Based on the relative change rate, the density change rate of each temperature point is obtained ;

[0017] By adopting the 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 piezoelectric ceramics, and the viscosity of the fluid is measured by the vibration method to obtain the viscosity. ;

[0018] Obtain the viscosity change rate at each temperature point based on the relative change rate ;

[0019] Get the flow point of the low temperature flow standard device ,in, , is an integer; by the formula Get the Reynolds number ,in, is the cross-sectional area of ​​the vibrating head of the probe rod; all Reynolds numbers are counted to obtain the Reynolds number range .

[0020] As a further improvement of the present invention, the specific analysis process of the flow characteristic analysis further includes:

[0021] Obtain the nominal flow range from the device technical documentation or nameplate ;Verify the effective flow range under current working conditions through actual testing , through the formula Get the effective index ;

[0022] The actual flow data obtained is analyzed, and the flow stability index is obtained by calculating the ratio of the standard deviation of the flow data to the extreme value difference of the flow data. ;

[0023] Extract the fluid density of the cryogenic flow standard device in a cryogenic environment and viscosity , through the formula Get the low temperature impact index ;in, The dimensional change rate caused by thermal expansion and contraction of the flow meter body and common pipeline materials; is the density value at the reference temperature; is the viscosity at the reference temperature; are the influencing weight factors of density, viscosity and dimensional change rate respectively;

[0024] Based on the output frequency of the frequency converter and the actual flow range, the frequency and flow characteristic curve is constructed by combining the least squares method. ;in, is the fitting coefficient, which is determined based on the measured data by the least squares method; is the frequency characteristic point; at the same time, through the formula Get the frequency conversion sensitivity index ,in, Indicates the flow rate change; Indicates the frequency change;

[0025] The effective index , flow stability index , low temperature impact index and frequency conversion sensitivity index After normalization, enter the formula Get the comprehensive index of flow characteristics ,in, They are the influence weight factors of effectiveness index, flow stability index, low temperature influence index and frequency conversion sensitivity index.

[0026] As a further improvement of the present invention, the specific implementation process of the frequency conversion strategy generation is as follows:

[0027] Based on the frequency and flow characteristic curve, combined with the expected expanded flow range , determine the frequency adjustment range of the frequency converter ;

[0028] By formula Get the minimum frequency ;

[0029] By formula Get the maximum frequency ;

[0030] in, is the reference frequency, and is the frequency adjustment factor, which is obtained based on the VFD capability, the physical limitations of the device, and the flow stability requirements; is the influencing weight factor of the comprehensive index of flow characteristics;

[0031] The frequency adjustment step is determined by the frequency conversion sensitivity index, and the frequency adjustment step is obtained by dividing the difference between the target flow rate and the initial flow rate by the frequency conversion sensitivity index. ;

[0032] The frequency change rate is determined based on the flow stability index, using the formula Get the actual adjustment rate; where, Adjust the rate based on the is the influencing weight factor of the flow stability index.

[0033] As a further improvement of the present invention, the specific implementation process of the flow range expansion includes:

[0034] The frequency converter gradually increases or decreases the output frequency at a constant speed based on the acceleration or deceleration parameters of the actual adjustment rate; the AC motor speed is controlled by adjusting the output frequency of the pump power supply, and the synchronous speed is obtained based on the asynchronous motor synchronous speed formula. ; Through the formula Calculate the slip rate ,in, is the actual 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 speed is consistent with the motor speed. Therefore, the pump speed is directly determined by the motor input frequency, establishing a linear proportional relationship between the pump speed and frequency.

[0035] During the process of increasing or decreasing the output frequency, the frequency adjustment step is used to ensure that the parameters are within the allowable range during the adjustment process. The temperature and pressure parameters of the fluid in the device are monitored in real time through temperature sensors and pressure sensors at fixed time intervals, and compared with the allowable temperature and pressure ranges for normal operation of the device.

[0036] When the temperature parameter or pressure parameter is not within the allowable pressure range, it is determined to be a parameter abnormality, and the actual adjustment rate of the output frequency is adjusted immediately. If the sampling data exceeds the parameter range for three consecutive times, the output frequency adjustment is suspended, the current frequency is kept unchanged, and the output frequency is gradually reduced at a decreasing rate of 0.5Hz / s until the temperature parameter and pressure parameter are restored to the normal range for five consecutive sampling data, and then the frequency adjustment is gradually resumed at a rate of 30% of the original deceleration.

[0037] As a further improvement of the present invention, the specific implementation process of the flow range expansion execution also includes:

[0038] During the frequency adjustment and flow expansion process, stability control measures are implemented, including using a PID control algorithm to maintain flow stability, and dynamically adjusting the control parameters based on the Reynolds number variation range; when the Reynolds number is less than the preset threshold, the proportional coefficient is reduced to avoid overshoot; when the Reynolds number is greater than the preset threshold, the differential effect is enhanced to suppress turbulent fluctuations; based on disturbance suppression technology, flow fluctuations caused by external interference are detected and suppressed; based on the curve switching strategy, a smooth transition is achieved between different frequency points to avoid sudden flow changes.

[0039] As a further improvement of the present invention, the specific verification steps of the extended result verification are as follows:

[0040] The expected expanded flow range , 7 test points are evenly selected based on the logarithmic law;

[0041] By using a Coriolis mass flowmeter to perform three repeated measurements for more than 120 seconds at each test point, the average value is taken as the actual flow value after eliminating transient data based on the Laida criterion. ;

[0042] By formula Get the test point error index ,in, To allow absolute error; is the low temperature compensation factor; is the target flow value;

[0043] When all test points meet the error index less than the preset threshold range, the expansion is determined to be successful, and the relevant parameters of the current frequency conversion controller are locked, including the frequency adjustment range, acceleration and deceleration;

[0044] When there is When the error index of the test point is greater than the preset threshold range, it is determined to be a local correction, and the test point with error is automatically located and PID parameter iterative optimization is performed;

[0045] When there are >3 test points with error indicators greater than the preset threshold range, the original settings are restored and the failure information is recorded to generate a log; after the verification is completed, all verification results are compiled into a standardized document and uploaded to the display terminal simultaneously.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] The present invention constructs a mapping relationship between temperature, density and viscosity, combines it with the analysis of the Reynolds number variation range, quantifies the impact of low-temperature environment on fluid characteristics, generates a comprehensive flow characteristic index, generates a frequency adjustment range based on the comprehensive flow characteristic index, and optimizes the adjustment step size and rate by combining the frequency conversion sensitivity index and the flow stability index. This effectively solves the problem of large measurement errors caused by fluctuations in physical parameters in traditional technologies and significantly improves the efficiency and stability of flow range expansion. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings;

[0049] Figure 1 Flow chart of the method of the present invention. DETAILED DESCRIPTION

[0050] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0051] It should be understood that the terms “include” and “comprising” used in the specification and claims of the present disclosure indicate the presence of described features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or collections thereof.

[0052] It should also be understood that the terminology used in this disclosure is for the purpose of describing specific embodiments only and is not intended to limit the disclosure. As used in this disclosure and the claims, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly indicates otherwise. It should further be understood that the term "and / or" as used in this disclosure and the claims refers to any and all possible combinations of one or more of the associated listed items, including and including these combinations.

[0053] See also Figure 1As shown, the present invention is a method for expanding the flow range of a low-temperature flow standard device based on variable frequency control, including: obtaining device operating parameters, analyzing flow characteristics, generating a variable frequency strategy, executing flow range expansion, and verifying the expansion effect.

[0054] S1. Acquisition of device operating parameters: The basic operating parameters of the low temperature flow standard device are acquired through the parameter acquisition unit, wherein the basic operating parameters of the low temperature flow standard device include: working medium temperature C, pressure B, density , viscosity As well as the upper and lower limits of the original flow range of the device, at the same time, the frequency conversion characteristic parameters of the flow standard device under different working conditions are obtained through the parameter acquisition unit;

[0055] S2. Flow characteristics analysis: The flow characteristics of the low-temperature flow standard device are analyzed. The specific analysis method is as follows:

[0056] 201. Extract the working medium temperature, pressure, density, viscosity, and the upper and lower limits of the device's original flow range from the basic operating parameters of the low-temperature flow standard device; and calculate the variation pattern of the working medium's physical properties at different temperatures, including the density change rate, viscosity change rate, and Reynolds number variation range;

[0057] 201-1. Density change rate: Get the lowest operating temperature of the low-temperature flow standard device from the parameter acquisition unit and maximum operating temperature ,based on The temperature is divided into equal intervals and an ordered array of temperatures is generated. ;Through the formula Calculate the temperature point ,in, , n is an integer; the temperature designed for the device under typical operating conditions is used as the reference temperature At the same time, obtain the fluid density parameters of the low-temperature flow standard device and build a mapping relationship between temperature and density ;Through the formula Calculate the density change rate at each temperature point ;in, is the density value at the temperature point; is the density value at the reference temperature;

[0058] 201-2. Viscosity change rate: By adopting a bypass sampling design, a sampling branch is drawn out from the side of the main pipeline, and the viscosity sensor is deployed on the sampling branch to avoid interference with the measurement caused by excessive flow velocity in the main pipeline; the viscosity sensor excites the probe rod to vibrate through piezoelectric ceramics. Changes in fluid viscosity will change the vibration frequency of the probe rod. Through the formula Calculate the viscosity ,in, is the fluid density; is the immersion depth of the probe vibrating head; is the cross-sectional area of ​​the vibrating head of the probe; is the no-load frequency; is the vibration frequency in the fluid;

[0059] By formula Calculate the viscosity change rate at each temperature point ;in, is the viscosity at the temperature point; is the viscosity at the reference temperature;

[0060] 201-3. Reynolds number variation range: Obtain the flow point of the low temperature flow standard device from the parameter acquisition unit ,in, , is an integer; by the formula Calculate the Reynolds number , calculate the Reynolds number under all working conditions to get the Reynolds number range ;

[0061] 202. Comprehensive analysis of flow characteristics:

[0062] 202-1. Obtain the nominal flow range from the device technical documentation or nameplate ;Verify the effective flow range under current working conditions through actual testing , through the formula Calculate the effective index , which indicates the degree of deviation between the basic performance of the device and the design specifications;

[0063] 202-2. The actual flow data obtained is analyzed for flow stability. Calculate the flow stability index ,in, For the The flow value of each sampling point, is the average flow rate, is the number of sampling points; the smaller the fluctuation of the flow stability index, the more stable the flow;

[0064] 202-3. Extract the fluid density of the low-temperature flow standard device in a low-temperature environment through the parameter acquisition unit and viscosity , through the formula Calculate the low temperature impact index ;in, The dimensional change rate caused by thermal expansion and contraction of the flow meter body and common pipeline materials; are the influencing weight factors of density, viscosity and dimensional change rate respectively;

[0065] 202-4. Based on the output frequency of the frequency converter and the actual flow range, the frequency and flow characteristic curve is constructed by combining the least squares method. ;in, is the fitting coefficient, which is determined based on the measured data by the least squares method; is the frequency characteristic point; at the same time, through the formula Calculate the frequency conversion sensitivity index ,in, Indicates the flow rate change; Indicates the frequency change;

[0066] 202-5, the effective index , flow stability index , low temperature impact index and frequency conversion sensitivity index After normalization, enter the formula Calculate the comprehensive index of flow characteristics ,in, They are the influence weight factors of effectiveness index, flow stability index, low temperature influence index and frequency conversion sensitivity index;

[0067] S3. Frequency conversion strategy generation: Based on the flow characteristics analysis results, design a suitable frequency conversion control strategy to achieve effective expansion of the flow range; the specific generation method is as follows:

[0068] 301. Determine the expansion target based on the frequency and flow characteristic curve and the expected flow range after expansion. , determine the frequency adjustment range of the frequency converter ;

[0069] By formula Calculate the minimum frequency ;

[0070] By formula Calculate the maximum frequency ;

[0071] in, is the reference frequency, and is the frequency adjustment factor, which is obtained based on the VFD capability, the physical limitations of the device, and the flow stability requirements; is the influencing weight factor of the comprehensive index of flow characteristics;

[0072] 302. Frequency conversion control parameter design:

[0073] Frequency adjustment step: The frequency adjustment step is determined based on the frequency conversion sensitivity index, and the formula Calculate the frequency adjustment step size, where is the initial flow rate, is the target flow;

[0074] Frequency adjustment rate: The frequency change rate is determined based on the flow stability index, and the formula Calculate the actual adjustment rate; where, Adjust the rate based on the is the influencing weight factor of the flow stability index;

[0075] S4. Flow range expansion execution: Based on the determined frequency adjustment range of the frequency converter, the output frequency of the frequency converter is adjusted to control the pump speed, thereby effectively expanding the flow range. The specific implementation process includes:

[0076] 401. Before performing the expansion operation, the low temperature flow standard device shall be prepared as follows:

[0077] Equipment inspection: confirm that the flow standard device and frequency conversion control system are working properly;

[0078] Parameter setting: load the generated frequency conversion strategy parameters into the frequency conversion controller;

[0079] Environmental stability: Ensure that the low-temperature environmental conditions are stable and the temperature fluctuations are within the allowable range;

[0080] Device preheating: Preheat the device based on the standard flow rate to allow each component to reach a stable working state, and record the benchmark flow value under the benchmark flow rate;

[0081] 402. Frequency adjustment execution: The frequency converter gradually increases or decreases the output frequency at a constant speed based on the acceleration or deceleration parameters of the actual adjustment rate; the frequency converter controls the AC motor speed by adjusting the pump power output frequency, and calculates the synchronous speed based on the asynchronous motor synchronous speed formula. , through the formula Calculate the slip rate in, is the actual speed; in variable frequency control, 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 speed is consistent with the motor speed. Therefore, the pump speed is directly determined by the motor input frequency, establishing a linear proportional relationship between the pump speed and frequency. ,in, is the pump speed, is the output frequency of the frequency converter, is the preset proportional coefficient; as the output frequency Increase or decrease of pump speed rise or fall accordingly;

[0082] During the process of output frequency reduction, the frequency adjustment step is based on the frequency to ensure that the parameters in the adjustment process are within the allowable range, and the temperature parameters and pressure parameters of the fluid in the device are monitored in real time; the temperature sensor and pressure sensor are used to collect data at fixed time intervals, and the collected temperature parameters and pressure parameters are compared with the allowable temperature range for normal operation of the device. and pressure range Make comparative judgments;

[0083] When the temperature parameter is not within the allowable range or the pressure parameter is not within the allowable pressure range, it is determined to be a parameter abnormality, and the actual adjustment rate of the output frequency is adjusted immediately. If the sampling data exceeds the parameter range for three consecutive times, the output frequency adjustment is suspended, the current frequency is kept unchanged, and the output frequency is gradually reduced at a decreasing rate of 0.5Hz / s until the temperature parameter and pressure parameter are restored to the normal range for five consecutive sampling data, and then the frequency adjustment is gradually resumed at a rate of 30% of the original deceleration rate;

[0084] 403. Stability control: During frequency adjustment and flow expansion, stability control measures shall be implemented. Specific control measures include:

[0085] By adopting a PID control algorithm to maintain flow stability, the control parameters are dynamically adjusted based on the range of Reynolds number variation. When the Reynolds number is less than a preset threshold, the fluid inertia is small, so the proportional coefficient is reduced to avoid overshoot. When the Reynolds number is greater than the preset threshold, the differential effect is enhanced to suppress turbulent fluctuations.

[0086] Detect and suppress flow fluctuations caused by external interference based on disturbance suppression technology; achieve smooth transition between different frequency points based on curve switching strategy to avoid sudden flow changes;

[0087] S5. Verification of expansion results: Verify the expansion results of the flow range to determine the expansion effect. The specific verification steps are as follows:

[0088] The expected expanded flow range , 7 test points are evenly selected based on the logarithmic law; through the formula Calculate the test points, where ,

[0089] They represent the lower limit, median value and upper limit of the expanded flow rate respectively, and the other test points evenly cover the expanded flow rate range;

[0090] Each test point was measured three times using a Coriolis mass flowmeter. Each measurement lasted longer than 120 seconds. The average value was taken as the actual flow value after excluding transient data based on the Laida criterion. ;

[0091] By formula Calculate the test point error index ,in, To allow absolute error; is the low temperature compensation factor; is the target flow value;

[0092] When all test points meet the error index less than the preset threshold range, the expansion is determined to be successful, and the relevant parameters of the current frequency conversion controller are locked, including the frequency adjustment range, acceleration and deceleration;

[0093] When there is When the error index of the test point is greater than the preset threshold range, it is determined to be a local correction, and the test point with error is automatically located and PID parameter iterative optimization is performed;

[0094] If the error index of more than three test points exceeds the preset threshold range, the expansion is judged to have failed and the abnormal fallback function is activated. By reading and restoring the original frequency setting of the device before the flow range expansion, the device is restored to the initial stable operating state. At the same time, detailed information of the expansion failure is recorded, including the failed test point, error value, and relevant parameters of the frequency converter. An error log is generated and uploaded to the display terminal. Based on the error log, technicians can determine the cause of the expansion failure and check and improve the relevant parameters of the frequency converter or the device hardware.

[0095] After the verification is completed, all verification results will be compiled into a standardized document. The document content includes a comparison table of flow ranges before and after expansion, measurement data and statistical analysis results of each test point, accuracy assessment report of the expansion area, frequency conversion parameter and flow relationship curve, recommended operating parameters and precautions, and long-term stability monitoring plan, and will be uploaded to the display terminal simultaneously.

[0096] The preferred embodiments of the present invention disclosed above are intended only to help illustrate the present invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the present invention to specific embodiments. Obviously, many modifications and variations are possible based on the contents of this specification. These embodiments are selected and described in detail in this specification to better explain the principles and practical applications of the present invention, thereby enabling those skilled in the art to better understand and utilize the present invention. The present invention is limited only 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: include: S1. Acquisition of device operating parameters: Acquisition of basic operating parameters of the low-temperature flow standard device and frequency conversion characteristic parameters under different operating conditions through the parameter acquisition unit; S2. Flow characteristics analysis: Analyze the flow characteristics of the low-temperature flow standard device; S3. Frequency conversion strategy generation: Based on the flow characteristics analysis results, design a suitable frequency conversion control strategy to achieve effective expansion of the flow range; S4, flow range expansion execution: Based on the determined frequency adjustment range of the frequency converter, the output frequency of the frequency converter is adjusted to control the pump speed, thereby achieving effective expansion of the flow range; S5. Verification of expansion results: Verify the expansion results of the flow range to determine the expansion effect; The specific analysis process of S2 includes: The working medium temperature, pressure, density, viscosity and the upper and lower limits of the original flow range of the device are extracted from the basic operating parameters of the low-temperature flow standard device to calculate the change law of the physical properties of the working medium at different temperatures; Obtain the lowest and highest operating temperatures of the low-temperature flow standard device, divide it based on the equal interval rule, generate an ordered array of temperatures, and obtain the temperature points; Based on the design temperature of the device under typical working conditions as the reference temperature; obtain the fluid density parameters of the low-temperature flow standard device and construct a mapping relationship between temperature and density ; Based on the relative change rate, the density change rate of each temperature point is obtained ; By adopting the 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 piezoelectric ceramics, 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 ; Get the flow point of the low temperature flow standard device ,in, , is an integer; by the formula Get the Reynolds number ,in, is the cross-sectional area of ​​the vibrating head of the probe rod; all Reynolds numbers are counted to obtain the Reynolds number range ; The specific analysis process of the flow characteristic analysis also includes: Obtain the nominal flow range from the device technical documentation or nameplate ;Verify the effective flow range under current working conditions through actual testing , through the formula Get the effective index ; The actual flow data obtained is analyzed, and the flow stability index is obtained by calculating the ratio of the standard deviation of the flow data to the extreme value difference of the flow data. ; Extract the fluid density of the cryogenic flow standard device in a cryogenic environment and viscosity , through the formula Get the low temperature impact index ;in, The dimensional change rate caused by thermal expansion and contraction of the flow meter body and common pipeline materials; is the density value at the reference temperature; is the viscosity at the reference temperature; are the influencing weight factors of density, viscosity and dimensional change rate respectively; Based on the output frequency of the frequency converter and the actual flow range, the frequency and flow characteristic curve is constructed by combining the least squares method. ;in, is the fitting coefficient, which is determined based on the measured data by the least squares method; is the frequency characteristic point; at the same time, through the formula Get the frequency conversion sensitivity index ,in, Indicates the flow rate change; Indicates the frequency change; The effective index , flow stability index , low temperature impact index and frequency conversion sensitivity index After normalization, enter the formula Get the comprehensive index of flow characteristics ,in, They are the influence weight factors of effectiveness index, flow stability index, low temperature influence index and frequency conversion sensitivity index.

2. The method for expanding the flow range of a low-temperature flow standard device based on variable frequency control according to claim 1, characterized in that: The specific implementation process of the frequency conversion strategy generation is as follows: Based on the frequency and flow characteristic curve, combined with the expected expanded flow range , determine the frequency adjustment range of the frequency converter ; By formula Get the minimum frequency ; By formula Get the maximum frequency ; in, is the reference frequency, and is the frequency adjustment factor, which is obtained based on the VFD capability, the physical limitations of the device, and the flow stability requirements; is the influencing weight factor of the comprehensive index of flow characteristics; The frequency adjustment step is determined by the frequency conversion sensitivity index, and the frequency adjustment step is obtained by dividing the difference between the target flow rate and the initial flow rate by the frequency conversion sensitivity index. ; The frequency change rate is determined based on the flow stability index, using the formula Get the actual adjustment rate; where, Adjust the rate based on the is the influencing weight factor of the flow stability index.

3. The method for expanding the flow range of a low-temperature flow standard device based on variable frequency control according to claim 1, characterized in that: The specific implementation process of the flow range expansion execution includes: The frequency converter gradually increases or decreases the output frequency at a constant speed based on the acceleration or deceleration parameters of the actual adjustment rate; the AC motor speed is controlled by adjusting the output frequency of the pump power supply, and the synchronous speed is obtained based on the asynchronous motor synchronous speed formula. ; Through the formula Calculate the slip rate ,in, is the actual 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 speed is consistent with the motor speed. Therefore, the pump speed is directly determined by the motor input frequency, establishing a linear proportional relationship between the pump speed and frequency. During the process of increasing or decreasing the output frequency, the frequency adjustment step is used to ensure that the parameters are within the allowable range during the adjustment process. The temperature and pressure parameters of the fluid in the device are monitored in real time through temperature sensors and pressure sensors at fixed time intervals, and compared with the allowable temperature and pressure ranges for normal operation of the device. When the temperature parameter or pressure parameter is not within the allowable pressure range, it is determined to be a parameter abnormality, and the actual adjustment rate of the output frequency is adjusted immediately. If the sampling data exceeds the parameter range for three consecutive times, the output frequency adjustment is suspended, the current frequency is kept unchanged, and the output frequency is gradually reduced at a decreasing rate of 0.5Hz / s until the temperature parameter and pressure parameter are restored to the normal range for five consecutive sampling data, and then the frequency adjustment is gradually resumed at a rate of 30% of the original deceleration.

4. The method for expanding the flow range of a low-temperature flow standard device based on variable frequency control according to claim 3, characterized in that: The specific implementation process of the flow range expansion execution also includes: During the frequency adjustment and flow expansion process, stability control measures are implemented, including using a PID control algorithm to maintain flow stability, and dynamically adjusting the control parameters based on the Reynolds number variation range; when the Reynolds number is less than the preset threshold, the proportional coefficient is reduced to avoid overshoot; when the Reynolds number is greater than the preset threshold, the differential effect is enhanced to suppress turbulent fluctuations; based on disturbance suppression technology, flow fluctuations caused by external interference are detected and suppressed; based on the curve switching strategy, a smooth transition is achieved between different frequency points to avoid sudden flow changes.

5. The method for expanding the flow range of a low-temperature flow standard device based on variable frequency control according to claim 1, characterized in that: The specific verification steps of the extended result verification are as follows: The expected expanded flow range , 7 test points are evenly selected based on the logarithmic law; By using a Coriolis mass flowmeter to perform three repeated measurements for more than 120 seconds at each test point, the average value is taken as the actual flow value after eliminating transient data based on the Laida criterion. ; By formula Get the test point error index ,in, To allow 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, the expansion is determined to be successful, and the relevant parameters of the current frequency conversion controller are locked, including the frequency adjustment range, acceleration and deceleration; When there is When the error index of the test point is greater than the preset threshold range, it is determined to be a local correction, and the test point with error is automatically located and PID parameter iterative optimization is performed; When there are >3 test points with error indicators greater than the preset threshold range, the original settings are restored and the failure information is recorded to generate a log; after the verification is completed, all verification results are compiled into a standardized document and uploaded to the display terminal simultaneously.

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