Gas pressure control device and method for direct absorption spectrum and gas analysis system

Abstract

The invention provides a gas pressure control device for a direct absorption spectrum, which belongs to the technical field of direct absorption spectrums.The device comprises an absorption cell, and the gas inlet end and the gas outlet end of the absorption cell are respectively provided with a pressure sensor and an MFC which are connected with a control monitoring unit; the pressure sensors and the MFCs at the air inlet end and the air outlet end monitor the pressure intensity and the flow of the air inlet end and the air outlet end in real time and transmit the pressure intensity and the flow to the control monitoring unit, the control monitoring unit comprises an outer ring PID controller and an inner ring PID controller, and the pressure intensity of the air inlet end, the pressure intensity of the air outlet end and the set target pressure intensity are input into the outer ring PID controller to obtain outer ring output values; the outer ring output value is input into an inner ring PID controller to obtain an inner ring output value, the gas inlet end MFC adjusts the flow of the gas inlet end according to the inner ring output value, and the gas outlet end MFC adjusts the flow of the gas outlet end according to the inner ring output value. And double-closed-loop PID control regulation is adopted, so that the accuracy and stability of pressure control in the gas absorption cell are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of direct absorption spectroscopy, and in particular to a gas pressure control device, method and gas analysis system for direct absorption spectroscopy. Background Art

[0002] Direct absorption spectroscopy is a spectroscopic analysis technique used to measure the concentration of specific molecules in a gas or liquid sample. When light passes through the sample, the molecules in the sample absorb light at specific wavelengths, resulting in a decrease in light intensity. By measuring the change in light intensity at different wavelengths, the concentration of specific molecules in the sample can be inferred.

[0003] In direct absorption spectroscopy, the pressure control of the absorption cell is very important because it directly affects the measurement accuracy and sensitivity. In current direct absorption spectroscopy techniques, a vacuum pump and a gas injection system are mostly used to adjust and control the pressure. The pressure in the absorption cell is reduced by the vacuum pump to remove the gas therein so that the pressure reaches a preset value. When precise pressure control is required, the target gas can be gradually introduced through a gas injection system (such as a mass flow controller or a needle valve) to adjust the pressure of the absorption cell to the required level. The system can monitor the pressure in real time through a pressure sensor and achieve closed-loop control by manually or automatically adjusting the gas introduction and exhaust rates. This method adjusts the opening degree of the valve manually or electrically to control the rate of gas inflow or outflow, thereby controlling the pressure of the absorption cell. Due to the limitations of the mechanical structure, the adjustment accuracy of this method is relatively low, and it is difficult to achieve high-precision control; the manual adjustment method requires manual intervention, with low efficiency and slow response speed. For low-pressure situations, it is difficult to achieve accurate pressure conditions; for measurement situations where gas needs to flow continuously, it is also difficult to maintain the stability of the pressure in the absorption cell, and it is impossible to avoid the influence of external factors such as mechanical vibration and temperature change on the pressure. Long-term use will also result in a decrease in accuracy due to mechanical wear of the valve.

[0004] In the prior art, in the Chinese patent application for invention "A Fluid Mixing Control System Based on a Dual-Closed-Loop PID Algorithm" with the publication number CN107479364A, in the first closed loop, the actual output water temperature is measured by a temperature sensor and feedback adjustment is performed using the PID algorithm. In the second closed loop, based on the data measured by the flow controller, the flow rate of water passing through the water pump and the voltage across the water pump are calculated, and then feedback is performed using the actual value measured by the flow sensor. Since the first and second closed loops of this system are two independent closed-loop PIDs, they cannot meet the high-precision requirements for pressure control of the gas absorption cell. Summary of the Invention

[0005] The technical problem to be solved by the present invention is how to improve the accuracy and stability of the pressure control in the gas absorption cell.

[0006] The present invention solves the above technical problems through the following technical solutions: A gas pressure control device for direct absorption spectroscopy. The device includes an absorption cell. A pressure sensor and an MFC connected to a control and monitoring unit are respectively provided at the inlet end and the outlet end of the absorption cell. The pressure sensors and MFCs at the inlet end and the outlet end respectively monitor the pressures and flows at the inlet end and the outlet end in real time and transmit them to the control and monitoring unit. The control and monitoring unit includes an outer-loop PID controller and an inner-loop PID controller. The inlet-end pressure, the outlet-end pressure, and the set target pressure are all input into the outer-loop PID controller to obtain an outer-loop output value. The outer-loop output value is input into the inner-loop PID controller to obtain an inner-loop output value. The inlet-end MFC adjusts the inlet-end flow according to the inner-loop output value, and the outlet-end MFC adjusts the outlet-end flow according to the inner-loop output value.

[0007] The present invention adopts double-loop PID control regulation. The outer-loop PID controller compares the actually measured pressure with the set pressure target value, first generates a desired flow value according to the pressure error, and takes the output of the outer-loop PID controller as the set target flow value and inputs it into the inner-loop PID controller to ensure that the system quickly reaches the set target pressure value. The inner-loop PID controller compares the actually measured flow with the set target flow value, calculates the inner-loop error, and calculates the control signal through the PID algorithm. According to this control signal, the MFCs at both ends are controlled to update the flow, and then the pressure is updated. The double-loop PID controller can more effectively adjust the system behavior, improve the accuracy and stability of the pressure control in the gas absorption cell, and enhance the response speed of the system.

[0008] Preferably, the inlet-end pressure and the set target pressure are input into the outer-loop PID controller. The inlet-end pressure is compared with the set target pressure to obtain a first pressure error. The first pressure error is calculated in the outer-loop PID controller to obtain a first outer-loop output value. The first outer-loop output value is used as the set target flow and input into the inner-loop PID controller. The inlet-end flow is compared with the set target flow to obtain a first flow error. The first flow error is calculated in the inner-loop PID controller to obtain a first inner-loop output value. The inlet-end MFC is adjusted according to the first inner-loop output value; the outlet-end pressure and the set target pressure are input into the outer-loop PID controller. The outlet-end pressure is compared with the set target pressure to obtain a second pressure error. The second pressure error is calculated in the outer-loop PID controller to obtain a second outer-loop output value. The second outer-loop output value is used as the set target flow and input into the inner-loop PID controller. The outlet-end flow is compared with the set target flow to obtain a second flow error. The second flow error is calculated in the inner-loop PID controller to obtain a second inner-loop output value. The outlet-end MFC is adjusted according to the second inner-loop output value.

[0009] Preferably, the outer-loop PID controller and the inner-loop PID controller respectively include proportional, integral, and differential operations.

[0010] The present invention also provides a gas pressure control method for direct absorption spectroscopy, and the method includes:

[0011] S1. The inlet pressure sensor and the outlet pressure sensor respectively and real-time monitor the inlet pressure and the outlet pressure of the absorption cell, and the inlet MFC and the outlet MFC respectively and real-time monitor the inlet flow rate and the outlet flow rate of the absorption cell;

[0012] S2. The inlet pressure and the set target pressure are input into the outer-loop PID controller. The inlet pressure is compared with the set target pressure to obtain a first pressure error. The first pressure error is calculated in the outer-loop PID controller to obtain a first outer-loop output value. The first outer-loop output value is used as the set target flow rate and input into the inner-loop PID controller. The inlet flow rate is compared with the set target flow rate to obtain a first flow error. The first flow error is calculated in the inner-loop PID controller to obtain a first inner-loop output value. The inlet MFC is adjusted according to the first inner-loop output value;

[0013] S3. The outlet pressure and the set target pressure are input into the outer-loop PID controller. The outlet pressure is compared with the set target pressure to obtain a second pressure error. The second pressure error is calculated in the outer-loop PID controller to obtain a second outer-loop output value. The second outer-loop output value is used as the set target flow rate and input into the inner-loop PID controller. The outlet flow rate is compared with the set target flow rate to obtain a second flow error. The second flow error is calculated in the inner-loop PID controller to obtain a second inner-loop output value. The outlet MFC is adjusted according to the second inner-loop output value.

[0014] Preferably, the process of calculating the first outer-loop output value from the first pressure error in S2 is the same as the process of calculating the second outer-loop output value from the second pressure error in S3, and includes:

[0015] S2.1. The first pressure error is respectively subjected to proportional, integral, and differential operations. The result P of the proportional operation out is:

[0016] P out = K p ·E

[0017] The result I of the integral operation out is:

[0018]

[0019] The result D of the differential operation out is:

[0020]

[0021] where K p 、K i 、Kd They are the proportional gain, integral gain, and derivative gain respectively. E is the first pressure error, and E k-1 , E k are the first pressure errors at the (k - 1)-th and k-th sampling moments respectively, and Δt is the sampling time;

[0022] S2.2. Calculate the first outer loop output value U out :

[0023]

[0024] Preferably, the process of calculating the first inner loop output value from the first flow error in the inner loop PID controller in S2 is the same as the process of calculating the second inner loop output value from the second flow error in the inner loop PID controller in S3, including:

[0025] S2.3. Calculate the first flow error E inner :

[0026] E inner = U out - PV inner

[0027] S2.4. Perform proportional, integral, and derivative operations on the first flow error respectively. The result P of the proportional operation inner is:

[0028] P inner = K p,inner · E inner

[0029] The result I of the integral operation inner is:

[0030]

[0031] The result D of the derivative operation inner is:

[0032]

[0033] where K p,inner , K i,inner , K d,inner are the proportional gain, integral gain, and derivative gain respectively, U out is the first outer loop output value, PV inner is the intake end flow rate, and E inner,k-1 , E inner,k are the first flow errors at the (k - 1)-th and k-th sampling moments respectively, and Δt is the sampling time;

[0034] S2.5. Calculate the first inner loop output value U inner :

[0035]

[0036] The present invention also provides a gas analysis system, including the gas pressure control device described above. The system further includes a pressure reducing valve, a stop valve, a buffer tank, a check valve, a turbomolecular pump, and a scroll pump. The pressure reducing valve, the stop valve, and the buffer tank are arranged in sequence upstream of the gas pressure control device along the gas flow direction. The check valve is located downstream of the gas pressure control device. The first output end of the check valve is connected to the turbomolecular pump, and the second output end of the check valve is connected to the scroll pump. The turbomolecular pump and the scroll pump are respectively connected to the control and monitoring unit.

[0037] Preferably, the working modes of the system include a stationary mode and a flow mode. Before selecting the working mode, first open the stop valve, the check valve, the inlet end MFC, and the outlet end MFC, close the pressure reducing valve, then turn on the scroll pump to pump to a low pressure, then turn off the scroll pump, turn on the turbomolecular pump to pump to a vacuum, and then turn on the pressure reducing valve to fill with the gas to be measured. When selecting the stationary mode, close the pressure reducing valve, and input the inlet end pressure, the outlet end pressure, and the set target pressure into the outer loop PID controller. When selecting the flow mode, directly input the inlet end pressure, the outlet end pressure, and the set target pressure into the outer loop PID controller.

[0038] Preferably, the system further includes a temperature control unit. The buffer tank and the absorption cell are respectively wrapped by heating films, and the outer layer of the heating films is wrapped by a heat insulation layer. A first temperature sensor is provided on the buffer tank, and a second temperature sensor is provided on the absorption cell. The heating films, the first temperature sensor, and the second temperature sensor are respectively connected to the temperature control unit.

[0039] Preferably, the temperature control unit adopts single-loop PID regulation. The temperature value monitored by the first temperature sensor is compared with the set target temperature to obtain the buffer tank temperature error. The buffer tank temperature error is input into the single-loop PID controller to calculate the first control signal. The temperature control unit adjusts the heating film on the buffer tank according to the first control signal. The temperature value monitored by the second temperature sensor is compared with the set target temperature to obtain the absorption cell temperature error. The absorption cell temperature error is input into the single-loop PID controller to calculate the second control signal. The temperature control unit adjusts the heating film on the absorption cell according to the second control signal.

[0040] The advantages provided by the present invention are as follows:

[0041] (1) The present invention adopts double - closed - loop PID control regulation. The outer - loop PID controller compares the actually measured pressure with the set pressure target value, first generates a desired flow value according to the pressure error, and takes the output of the outer - loop PID controller as the set target flow value and inputs it into the inner - loop PID controller to ensure that the system quickly reaches the set target pressure value. The inner - loop PID controller compares the actually measured flow with this set target flow value, calculates the inner - loop error, and calculates the control signal through the PID algorithm. According to this control signal, the MFCs at both ends are controlled to update the flow, and then the pressure is updated. The double - closed - loop PID controller can more effectively adjust the system behavior, improve the accuracy and stability of pressure control in the gas absorption cell, and enhance the response speed of the system.

[0042] (2) In the present invention, a pressure sensor and an MFC are respectively arranged at the inlet end and the outlet end of the absorption cell, which can simultaneously monitor the pressures at both ends of the absorption cell, control the flows at both ends, realize double - closed - loop PID control, perform real - time feedback on the errors generated at both ends, generate corresponding control signals, and make the MFC execute the operation of increasing or decreasing the flow, thereby realizing a stable pressure control effect under flowing conditions. At the same time, in the case of static pressure control, the double - closed - loop PID will also generate feedback according to the pressure fluctuation, control the MFCs at both ends to open or close slightly, and compensate for the pressure.

[0043] (3) In the gas analysis system of the present invention, a buffer cell is arranged upstream of the absorption cell. The temperature sensors and heating films on the buffer cell and the absorption cell are respectively connected to the temperature control unit, and the temperature control unit is used to respectively control and adjust the temperatures in the buffer cell and the absorption cell. While realizing stable gas flow, it can reduce the influence of external temperature fluctuations on the pressure in the absorption cell.

[0044] (4) In the gas analysis system of the present invention, the scroll pump and the turbo - molecular pump are used in combination. Before using the turbo - molecular pump, the scroll pump quickly discharges the gas in the pipeline, which can achieve a higher vacuum degree and working efficiency, has smaller mechanical vibration, and reduces the influence on the system optical path under continuous monitoring. Description of the Drawings

[0045] Figure 1 is a schematic diagram of the gas analysis system provided by the embodiment of the present invention;

[0046] Figure 2 is a schematic diagram of the gas pressure control method for direct absorption spectroscopy provided by the embodiment of the present invention;

[0047] Figure 3 is a working flow chart of the gas analysis system provided by the embodiment of the present invention;

[0048] Figure 4Schematic diagram of the temperature control unit in the gas analysis system provided by the embodiments of the present invention;

[0049] In the figure: 1 gas cylinder, 2 pressure reducing valve, 3 pipeline, 4 stop valve, 5 heating film, 6 buffer pool, 7 heat preservation layer, 8 first temperature sensor, 9 first mass flow controller, 10 first pressure sensor, 11 temperature control unit, 12 absorption cell, 13 second temperature sensor, 14 second pressure sensor, 15 second mass flow controller, 16 check valve, 17 turbo molecular pump, 18 scroll pump, 19 serial port line, 20 control and monitoring unit. Specific embodiments

[0050] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following describes the technical solutions of the present invention clearly and completely with reference to specific embodiments and the accompanying drawings. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. 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 scope of protection of the present invention.

[0051] Embodiment 1

[0052] As Figure 1 and Figure 2As shown in the figure, this embodiment provides a gas pressure control device for direct absorption spectroscopy, including: a first mass flow controller 9 (inlet MFC), a first pressure sensor 10 (inlet pressure sensor), an absorption cell 12, a second pressure sensor 14 (outlet pressure sensor), and a second mass flow controller 15 (outlet MFC) arranged in sequence on the pipeline 3 along the gas flow direction. The first mass flow controller 9, the first pressure sensor 10, the second pressure sensor 14, and the second mass flow controller 15 are respectively connected to the control and monitoring unit 20 through a serial cable 19. The first pressure sensor 10 and the first mass flow controller 9 respectively monitor the inlet pressure and the inlet flow rate of the absorption cell 12 in real time. The second pressure sensor 14 and the second mass flow controller 15 respectively monitor the outlet pressure and the outlet flow rate of the absorption cell 12 in real time. The inlet pressure, flow rate, outlet pressure, and flow rate are all transmitted to the control and monitoring unit 20. The inlet pressure is compared with the set target pressure to obtain a first pressure error, and the outlet pressure is compared with the set target pressure to obtain a second pressure error. The first pressure error is input into the outer-loop PID controller to obtain a first outer-loop output value, and the first outer-loop output value is used as the set target flow rate and input into the inner-loop PID controller. The inlet flow rate is compared with the set target flow rate to obtain a first flow error, and the outlet flow rate is compared with the set target flow rate to obtain a second flow error. The first flow error undergoes proportional P, integral I, and derivative D operations in the inner-loop PID controller to obtain a first inner-loop output value. According to the first inner-loop output value, the valve opening of the first mass flow controller 9 is adjusted to achieve the adjustment of the inlet flow rate, and further achieve the adjustment of the inlet pressure. The second pressure error is input into the outer-loop PID controller to obtain a second outer-loop output value, and the second outer-loop output value is used as the set target flow rate and input into the inner-loop PID controller. The outlet flow rate is compared with the set target flow rate to obtain a second flow error. The second flow error undergoes proportional P, integral I, and derivative D operations in the inner-loop PID controller to obtain a second inner-loop output value. According to the second inner-loop output value, the valve opening of the second mass flow controller 15 is adjusted to achieve the adjustment of the outlet flow rate, and further achieve the adjustment of the outlet pressure. Among them, the mass flow controller can be abbreviated as MFC, and its full name is Mass Flow Controller.

[0053] The absorption cell 12 is used to hold the gas to be measured. Inside the absorption cell 12, the gas to be measured interacts with the light source. The absorption cell allows the light beam to pass through the gas sample. After the light interacts with the gas molecules, the change in light intensity is used to analyze the gas composition and concentration. The absorption cell needs to have good sealing performance to prevent gas leakage and the entry of external gases, ensuring the accuracy of the measurement. The temperature and pressure inside the absorption cell should be kept constant to obtain stable and reproducible measurement results, and it is the main object of temperature and pressure control.

[0054] The first pressure sensor 10 is located at the gas inlet end of the absorption cell 12, and the second pressure sensor 14 is located at the gas outlet end of the absorption cell 12. The consistency of the values of the two pressure sensors can be used to ensure the stability of the gas in the absorption cell. Under the condition of flow control, the pressure sensors at two different positions can ensure the balance between the gas inlet end and the gas outlet end. Both pressure sensors are high-precision pressure sensors, such as capacitive sensors, piezoresistive sensors, piezoelectric sensors, or fiber optic sensors. Their measurement principle is to convert physical pressure into an electrical signal to achieve pressure measurement. Select a suitable pressure sensor according to the actual situation to achieve better pressure control effect while saving budget. During long-term use, the output of the sensor should remain stable with little drift. Select a sensor with a shorter response time suitable for dynamic pressure monitoring. Select a suitable measurement range according to the application requirements to ensure the normal operation of the sensor in the working environment. Calibrate the sensor regularly to ensure the measurement accuracy. By reasonably selecting and using high-precision pressure sensors, the pressure monitoring ability of the precise pressure control system can be significantly improved, ensuring the stability and reliability of the system.

[0055] The control and monitoring unit 20 includes a hardware part and a software part. The hardware part includes an industrial control computer and a serial port, and the software part includes a monitoring unit and a control unit. The monitoring unit is respectively connected to the first pressure sensor 9 and the second pressure sensor 15 through the serial cable 19, and the control unit is respectively connected to the first mass flow controller 10 and the second mass flow controller 14 through the serial cable. The control and monitoring unit can communicate with the double closed-loop PID algorithm to send the real-time monitored data to the PID controller to obtain corresponding feedback for control. The control and monitoring unit has a graphical interface for displaying real-time data and control parameters, and it also provides a network interface to enable remote monitoring and control. In the present invention, each part of the system is connected through the RS485 serial cable and the serial port control program in the industrial control computer, realizing the integration of pressure control and data acquisition. Adjusting the gas in the absorption cell to the required pressure and controlling the pressure to maintain a stable value do not require manual adjustment of each component, with a high degree of automation.

[0056] The present invention adopts double - closed - loop PID control regulation. The outer - loop PID controller compares the actually measured pressure with the set pressure target value, first generates a desired flow value according to the pressure error, and takes the output of the outer - loop PID controller as the set target flow value and inputs it into the inner - loop PID controller to ensure that the system quickly reaches the set target pressure value. The inner - loop PID controller compares the actually measured flow with the set target flow value, calculates the inner - loop error, and calculates the control signal through the PID algorithm to drive the actuator. In the present invention, the flow value is updated by controlling the opening degree of the mass flow control, and then the pressure value is updated. The output of the inner - loop PID controller will affect the actual output of the system, thereby changing the feedback values (the inlet - end pressure and the outlet - end pressure monitored in real time) of the outer - loop PID controller, and recalculating the outputs of the outer - loop PID controller and the inner - loop PID controller in the next control cycle. The whole process is continuously iterated to achieve the dynamic control of the system. The double - closed - loop PID controller can more effectively adjust the system behavior, improve the accuracy and stability of the pressure control in the gas absorption cell, and enhance the response speed of the system. Since the inlet - end and the outlet - end regulate and control the flow based on the same set target pressure, the consistency of the pressure between the inlet - end and the outlet - end of the absorption cell can be ensured.

[0057] The rapid adjustment of the inner loop ensures that the system can quickly respond to any disturbance and reduce pressure fluctuations. The outer loop provides a higher - level control to ensure that the system operates stably within the set pressure range. Through the double - closed - loop structure, the system can better handle the non - linearity and dynamic changes in the system and improve the control accuracy.

[0058] Embodiment 2

[0059] This embodiment provides a gas pressure control method for direct absorption spectroscopy. The method includes the following steps:

[0060] S1. The inlet - end pressure sensor and the outlet - end pressure sensor respectively monitor the inlet - end pressure and the outlet - end pressure of the absorption cell in real time, and the inlet - end MFC and the outlet - end MFC respectively monitor the inlet - end flow and the outlet - end flow of the absorption cell in real time;

[0061] S2. The inlet - end pressure and the set target pressure are input into the outer - loop PID controller. The inlet - end pressure is compared with the set target pressure to obtain the first pressure error. The first pressure error is calculated in the outer - loop PID controller to obtain the first outer - loop output value. The first outer - loop output value is used as the set target flow and input into the inner - loop PID controller. The inlet - end flow is compared with the set target flow to obtain the first flow error. The first flow error is calculated in the inner - loop PID controller to obtain the first inner - loop output value. The inlet - end MFC is adjusted according to the first inner - loop output value;

[0062] S3. The pressure at the outlet end and the set target pressure are input into the outer-loop PID controller. The pressure at the outlet end is compared with the set target pressure to obtain the second pressure error. The second pressure error is calculated in the outer-loop PID controller to obtain the second outer-loop output value. The second outer-loop output value is used as the set target flow rate and input into the inner-loop PID controller. The flow rate at the outlet end is compared with the set target flow rate to obtain the second flow error. The second flow error is calculated in the inner-loop PID controller to obtain the second inner-loop output value. The outlet-end MFC is adjusted according to the second inner-loop output value.

[0063] The process of calculating the first outer-loop output value from the first pressure error in S2 in the outer-loop PID controller is the same as the process of calculating the second outer-loop output value from the second pressure error in S3 in the outer-loop PID controller, including:

[0064] S2.1. The first pressure error is subjected to proportional, integral, and differential operations respectively. The result P out of the proportional operation is:

[0065] P out = K p ·E

[0066] The result I out of the integral operation is:

[0067]

[0068] The result D out of the differential operation is:

[0069]

[0070] where K p , K i , K d are the proportional gain, integral gain, and differential gain respectively, E is the first pressure error, E k-1 , E k are the first pressure errors at the (k - 1)-th and k-th sampling moments respectively, and Δt is the sampling time;

[0071] S2.2. Calculate the first outer-loop output value U out :

[0072]

[0073] The process of calculating the first inner-loop output value from the first flow error in S2 in the inner-loop PID controller is the same as the process of calculating the second inner-loop output value from the second flow error in S3 in the inner-loop PID controller, including:

[0074] S2.3. Calculate the first flow error E inner :

[0075] E inner = U out - PV inner

[0076] S2.4. Perform proportional, integral, and derivative operations on the first flow error respectively. The result of the proportional operation P inner is:

[0077] P inner = K p,inner ·E inner

[0078] The result of the integral operation I inner is:

[0079]

[0080] The result of the derivative operation D inner is:

[0081]

[0082] Among them, K p,inner , K i,inner , K d,inner are the proportional gain, integral gain, and derivative gain respectively. U out is the output value of the first outer loop, PV inner is the flow rate at the intake end, E inner,k-1 , E inner,k are the first flow errors at the (k - 1)-th and k-th sampling moments respectively, and Δt is the sampling time;

[0083] S2.5. Calculate the output value U inner of the first inner loop:

[0084]

[0085] In the present invention, by simultaneously monitoring the pressures at both ends of the absorption cell and controlling the flow rates at both ends, double closed-loop PID control is achieved. The errors generated at both ends are fed back in real time to generate corresponding control signals, causing the MFC to perform operations of increasing or decreasing the flow rate, thereby achieving a stable pressure control effect under flowing conditions. At the same time, in the case of static pressure control, the double closed-loop PID will also generate feedback according to the pressure fluctuations, controlling the MFCs at both ends to open or close slightly to compensate for the pressure.

[0086] K p , K i , K d , K p,inner , K i,inner , K d,innerThe adjustment of these gains is crucial as they determine the stability and response speed of the system. Usually, it is necessary to find the optimal gain values through experiments and adjustments. Inappropriate gains may cause the system to oscillate or even become unstable.

[0087] The output value U of the first inner loop inner As a control signal to drive the opening degree of the MFC at the intake end. Similarly, the output value of the second inner loop is used as a control signal to drive the opening degree of the MFC at the outlet end, to update the flow rate and further update the pressure. The relationship between the opening degree u of the MFC, the flow rate Q actual and the pressure P actual is as follows:

[0088] Q actual = m·u

[0089] P actual = k·Q actual

[0090] where m is the mass of the gas to be measured.

[0091] Embodiment 3

[0092] Continue to refer to Figure 1 , the difference between this embodiment and Embodiment 1 is that: this embodiment provides a gas analysis system, including the gas pressure control device of Embodiment 1, and also includes a gas cylinder 1, a pressure reducing valve 2, a stop valve 4, a buffer tank 6, a check valve 16, a turbomolecular pump 17, and a scroll pump 18. The gas cylinder 1 is located at one end of the pipeline 3. The pressure reducing valve 2, the stop valve 4, and the buffer tank 6 are arranged in sequence along the gas flow direction upstream of the pressure control device. The check valve 16 is located downstream of the second mass flow controller 15. The first output end of the check valve 16 is connected to the turbomolecular pump 17, and the second output end of the check valve 16 is connected to the scroll pump 18. The buffer tank 6 and the absorption cell 12 are respectively wrapped by a heating film 5, and the outer layer of the heating film 5 is wrapped by a heat insulation layer 7. A first temperature sensor 8 is provided on the buffer tank 6, and a second temperature sensor 13 is provided on the absorption cell 12. The heating film 5, the first temperature sensor 8, and the second temperature sensor 13 are respectively connected to a temperature control unit 11. The turbomolecular pump 17 and the scroll pump 18 are respectively connected to a control and monitoring unit 20 through a serial cable 19.

[0093] The gas cylinder 1 is a high-pressure gas cylinder for storing the gas to be measured. The material can be selected from steel, stainless steel or aluminum alloy, and the capacity is determined according to requirements. The gas cylinder 1 is equipped with a safety valve and a pressure gauge. The safety valve is used to prevent overpressure, and the pressure gauge is used to monitor the pressure inside the gas cylinder in real time. Standardized interfaces are used to facilitate the connection of the gas cylinder to the pressure reducing valve 2 and the pipeline 3. The pressure reducing valve 2 is used to reduce the gas pressure in the gas cylinder 1 to a controllable range to provide a stable output pressure. In the present invention, a single-stage or double-stage pressure reducing valve is selected. The double-stage pressure reducing valve can provide a more stable output. The material of the pressure reducing valve is selected according to the characteristics of the gas to be measured to be corrosion-resistant, such as stainless steel. The pipeline 3 is an airtight pipeline, and the material is selected from corrosion-resistant materials, such as stainless steel or PTFE, to ensure airtightness and durability. High-quality quick connectors and sealing rings are used for the pipeline to facilitate installation and maintenance. In the pipeline layout, the number of bends and joints needs to be reduced to reduce the risk of gas leakage.

[0094] The stop valve 4 is used to open or close the gas flow and is used for the start-stop control of the system. By manually operating the opening and closing of the stop valve, it is suitable for occasions where frequent operation is not required. The material of the stop valve can be selected from stainless steel, brass or plastic according to the characteristics of the gas to be measured and the use environment.

[0095] The heating film 5 is a temperature-controlled heating film, which is a thin-film electric heating element. Heat is generated by current flowing through the conductive material. Its working principle is based on the resistance heating effect, that is, when current passes through a conductor, due to the existence of resistance, electrical energy is converted into heat energy. The heating film 5 includes a substrate, a conductive layer and an insulating layer. The substrate can be made of polyimide, polyester or other materials with high temperature resistance and good insulation performance. The choice of the substrate affects the flexibility and durability of the heating film. The conductive layer is selected from metal foil or conductive ink. The thickness and pattern design of the conductive layer determine the resistance value and heating uniformity of the heating film. An insulating material is covered on the conductive layer to prevent current leakage and short circuit. The insulating layer is usually integrated with the substrate to provide double protection. In the precise pressure control system, the heating film is used to maintain a constant temperature of the gas or equipment to prevent temperature fluctuations from affecting the measurement results. In an environment with large temperature changes, the heating film is used to compensate for the influence of temperature changes on the system to ensure measurement accuracy.

[0096] The buffer pool 6 is located upstream of the first mass flow controller 9. The gas about to enter the absorption cell 12 is preheated through the temperature control unit 11 and the heating film 5, which can reduce the influence of temperature fluctuations on the pressure, and is used to stabilize the flow of the gas to be measured to reduce pressure fluctuations. The buffer pool is made of pressure-resistant material, and its volume is determined according to requirements.

[0097] The thermal insulation layer 7 reduces the heat exchange between the system and the external environment through heat-insulating materials, maintains the temperature stability inside the system, reduces heat loss, and selects appropriate thermal insulation layer materials and thickness according to the operating temperature of the equipment and environmental conditions to achieve the best thermal insulation effect. Ensure the tight fit of the thermal insulation material to avoid the generation of voids and heat bridges. Use appropriate fixing and sealing materials to ensure the stability and durability of the thermal insulation layer. Regularly check the integrity of the thermal insulation layer and repair damaged or aged parts in a timely manner.

[0098] Both the first temperature sensor 8 and the second temperature sensor 13 are platinum resistance temperature sensors. The temperature sensor using platinum as the sensing material has good linearity, high precision, and strong stability.

[0099] The temperature control unit 11 is dual-channel, providing temperature setting and monitoring functions, supporting multiple control algorithms, and having good stability and response speed. Through a closed-loop control system, continuously monitor and adjust the temperature to ensure that the system is stable at the set value. The PID controller is a commonly used choice because it can provide precise temperature control, independent of the dual closed-loop PID system used for pressure control. High control precision, fast response time, and stability during long-term use are all important indicators for selecting the temperature control module. As Figure 4 shown, the temperature control unit 11 adopts single-loop PID regulation. The temperature value monitored by the first temperature sensor 8 is compared with the set target temperature to obtain the buffer pool temperature error. The buffer pool temperature error is input into the single-loop PID controller to calculate the first control signal. The temperature control unit 11 adjusts the output of the heating film 5 on the buffer pool 6 according to the first control signal. The temperature value monitored by the second temperature sensor 13 is compared with the set target temperature to obtain the absorption pool temperature error. The absorption pool temperature error is input into the single-loop PID controller to calculate the second control signal. The temperature control unit 11 adjusts the output of the heating film 5 on the absorption pool 12 according to the second control signal.

[0100] In the present invention, by arranging a buffer pool upstream of the absorption pool, the temperature sensors and heating films on the buffer pool and the absorption pool are respectively connected to the temperature control unit, and the temperature control of the buffer pool and the absorption pool is respectively realized through the temperature control unit, which can achieve stable gas flow while reducing the influence of external temperature fluctuations on the pressure inside the absorption pool.

[0101] The check valve 16 is used to prevent the gas in the downstream gas pump from flowing back, avoid interfering with the gas in the absorption pool, and protect the safety of the system. The turbo molecular pump 17 or the scroll pump 18 can be manually selected. The turbo molecular pump 17 uses the relative movement between the high-speed rotating rotor blades and the stator blades to transfer momentum to gas molecules, thereby compressing and discharging the gas molecules step by step. Its working principle is based on the momentum transfer of molecular flow and is suitable for high vacuum and ultra-high vacuum environments, and the vacuum degree can reach 10 -8Torr or lower, then use the scroll pump 18 to pump the absorption cell to a low pressure and further pump the absorption cell to a vacuum. During the pressure control process, continuously extract the gas in the gas path to provide a negative pressure at the gas outlet end. The turbomolecular pump 17 has smaller mechanical vibrations and has less impact on the optical path part of the system during long-term monitoring. The scroll pump 18 compresses and discharges gas through the relative movement of two meshing scroll disks. One scroll disk is fixed, and the other scroll disk orbits around it, forming multiple gas compression chambers. The vacuum degree can usually reach 10 -3 Torr. Before using the turbomolecular pump, quickly discharge the gas in the pipeline through the scroll pump. The scroll pump and the turbomolecular pump are used in combination to achieve a higher vacuum degree and working efficiency. The present invention uses both the turbomolecular pump and the scroll pump, which can generate better vacuum conditions faster, has smaller mechanical vibrations, and reduces the impact on the optical path of the system under continuous monitoring.

[0102] As Figure 3 shown, the working modes of the system include a stationary mode and a flow mode. Before selecting the working mode, first open the stop valve, check valve, inlet MFC, and outlet MFC, close the pressure reducing valve, then turn on the scroll pump to pump to a low pressure, then turn off the scroll pump, turn on the turbomolecular pump to pump to a vacuum, and then turn on the pressure reducing valve to fill the gas to be measured. When selecting the stationary mode, close the pressure reducing valve and input the inlet pressure, outlet pressure, and set target pressure into the outer loop PID controller. When selecting the flow mode, directly input the inlet pressure, outlet pressure, and set target pressure into the outer loop PID controller. Compare the inlet pressure with the set target pressure. When the inlet pressure is less than the set target pressure, the inlet MFC increases the inlet flow rate according to the output value of the inner loop. When the inlet pressure is greater than the set target pressure, the inlet MFC decreases the inlet flow rate according to the output value of the inner loop. Similarly, when the outlet pressure is less than the set target pressure, the outlet MFC increases the outlet flow rate according to the output value of the inner loop. When the outlet pressure is greater than the set target pressure, the outlet MFC decreases the outlet flow rate according to the output value of the inner loop.

[0103] The above embodiments are only used to illustrate the technical solutions of the present invention, not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A gas pressure control device for direct absorption spectroscopy, characterized in that: The device includes an absorption cell. A pressure sensor and an MFC connected to the control and monitoring unit are respectively provided at the inlet end and the outlet end of the absorption cell. The pressure sensors and the MFCs at the inlet end and the outlet end respectively monitor the pressure and flow rate at the inlet end and the outlet end in real time and transmit them to the control and monitoring unit. The control and monitoring unit includes an outer-loop PID controller and an inner-loop PID controller. The inlet-end pressure, the outlet-end pressure and the set target pressure are all input into the outer-loop PID controller to obtain an outer-loop output value. The outer-loop output value is input into the inner-loop PID controller to obtain an inner-loop output value. The inlet-end MFC adjusts the inlet-end flow rate according to the inner-loop output value, and the outlet-end MFC adjusts the outlet-end flow rate according to the inner-loop output value.

2. The gas pressure control device for direct absorption spectroscopy according to claim 1, characterized in that: The inlet-end pressure and the set target pressure are input into the outer-loop PID controller. The inlet-end pressure is compared with the set target pressure to obtain a first pressure error. The first pressure error is calculated in the outer-loop PID controller to obtain a first outer-loop output value. The first outer-loop output value is used as the set target flow rate and input into the inner-loop PID controller. The inlet-end flow rate is compared with the set target flow rate to obtain a first flow error. The first flow error is calculated in the inner-loop PID controller to obtain a first inner-loop output value. The inlet-end MFC is adjusted according to the first inner-loop output value; The outlet-end pressure and the set target pressure are input into the outer-loop PID controller. The outlet-end pressure is compared with the set target pressure to obtain a second pressure error. The second pressure error is calculated in the outer-loop PID controller to obtain a second outer-loop output value. The second outer-loop output value is used as the set target flow rate and input into the inner-loop PID controller. The outlet-end flow rate is compared with the set target flow rate to obtain a second flow error. The second flow error is calculated in the inner-loop PID controller to obtain a second inner-loop output value. The outlet-end MFC is adjusted according to the second inner-loop output value.

3. The gas pressure control device for direct absorption spectroscopy according to claim 2, characterized in that: The outer-loop PID controller and the inner-loop PID controller respectively include proportional, integral and derivative operations.

4. A gas pressure control method for direct absorption spectroscopy, characterized in that: the method It includes: S1. The inlet-end pressure sensor and the outlet-end pressure sensor respectively monitor the inlet-end pressure and the outlet-end pressure of the absorption cell in real time, and the inlet-end MFC and the outlet-end MFC respectively monitor the inlet-end flow rate and the outlet-end flow rate of the absorption cell in real time; S2. The inlet-end pressure and the set target pressure are input into the outer-loop PID controller. The inlet-end pressure is compared with the set target pressure to obtain a first pressure error. The first pressure error is calculated in the outer-loop PID controller to obtain a first outer-loop output value. The first outer-loop output value is used as the set target flow rate and input into the inner-loop PID controller. The inlet-end flow rate is compared with the set target flow rate to obtain a first flow error. The first flow error is calculated in the inner-loop PID controller to obtain a first inner-loop output value. The inlet-end MFC is adjusted according to the first inner-loop output value; S3. The pressure at the outlet end and the set target pressure are input into the outer-loop PID controller. The pressure at the outlet end is compared with the set target pressure to obtain the second pressure error. The second pressure error is calculated in the outer-loop PID controller to obtain the second outer-loop output value. The second outer-loop output value is used as the set target flow rate and input into the inner-loop PID controller. The flow rate at the outlet end is compared with the set target flow rate to obtain the second flow error. The second flow error is calculated in the inner-loop PID controller to obtain the second inner-loop output value. The outlet end MFC is adjusted according to the second inner-loop output value.

5. The gas pressure control method for direct absorption spectroscopy according to claim 4, characterized in that: The process of calculating the first outer-loop output value from the first pressure error in the outer-loop PID controller in S2 is the same as the process of calculating the second outer-loop output value from the second pressure error in the outer-loop PID controller in S3, including: S2.

1. Perform proportional, integral, and differential operations on the first pressure error respectively. The result P of the proportional operation out is as follows: P out = K p · E The result I of the integral operation out is as follows: I out = K i ·∫Edt, The result D of the differential operation out is as follows: Among them, K p , K i , K d are the proportional gain, integral gain, and derivative gain respectively, E is the first pressure error, and E k-1 , E k are the first pressure errors at the (k - 1)-th and k-th sampling instants respectively, and Δt is the sampling time; S2.

2. Calculate the first outer loop output value U out :

6. The gas pressure control method for direct absorption spectroscopy according to claim 4, characterized in that: The process of calculating the first inner-loop output value from the first flow error in the inner-loop PID controller in S2 is the same as the process of calculating the second inner-loop output value from the second flow error in the inner-loop PID controller in S3, including: S2.

3. Calculate the first flow error E inner : E inner = U out - PV inner S2.

4. Perform proportional, integral, and derivative operations on the first flow error respectively. The result P of the proportional operation inner is as follows: P inner = K p,inner · E inner The result I of the integration operation inner is as follows: The result D of the differential operation inner is as follows: Among them, K p,inner , K i,inner , K d,inner are the proportional gain, integral gain, and derivative gain respectively. U out is the output value of the first outer loop, PV inner is the flow rate at the intake end, E inner,k-1 , E inner,k are the first flow rate errors at the (k - 1)-th and k-th sampling moments respectively, and Δt is the sampling time; S2.

5. Calculate the output value U of the first inner loop inner :

7. A gas analysis system, comprising the gas pressure control device according to any one of claims 1-3, characterized in that: The system further includes a pressure reducing valve, a stop valve, a buffer tank, a check valve, a turbomolecular pump, and a scroll pump. The pressure reducing valve, the stop valve, and the buffer tank are arranged in sequence along the gas flow direction upstream of the gas pressure control device. The check valve is located downstream of the gas pressure control device. The first output end of the check valve is connected to the turbomolecular pump, and the second output end of the check valve is connected to the scroll pump. The turbomolecular pump and the scroll pump are respectively connected to the control and monitoring unit.

8. The gas analysis system according to claim 7, wherein: The working modes of the system include a stationary mode and a flow mode. Before selecting the working mode, first open the stop valve, the check valve, the inlet end MFC, and the outlet end MFC, close the pressure reducing valve, then turn on the scroll pump to pump to a low pressure, then turn off the scroll pump, turn on the turbomolecular pump to pump to a vacuum, and then turn on the pressure reducing valve to fill with the gas to be measured. When the stationary mode is selected, close the pressure reducing valve, and input the inlet end pressure, the outlet end pressure, and the set target pressure into the outer-loop PID controller. When the flow mode is selected, directly input the inlet end pressure, the outlet end pressure, and the set target pressure into the outer-loop PID controller.

9. The gas analysis system according to claim 7, characterized in that: The system further includes a temperature control unit. The buffer tank and the absorption cell are respectively wrapped by heating films, and the outer layer of the heating films is wrapped by a heat insulation layer. A first temperature sensor is provided on the buffer tank, and a second temperature sensor is provided on the absorption cell. The heating films, the first temperature sensor, and the second temperature sensor are respectively connected to the temperature control unit.

10. The gas analysis system according to claim 7, wherein: The temperature control unit adopts single-loop PID regulation. The temperature value monitored by the first temperature sensor is compared with the set target temperature to obtain the buffer tank temperature error. The buffer tank temperature error is input into the single-loop PID controller to calculate the first control signal. The temperature control unit adjusts the heating film on the buffer tank according to the first control signal. The temperature value monitored by the second temperature sensor is compared with the set target temperature to obtain the absorption cell temperature error. The absorption cell temperature error is input into the single-loop PID controller to calculate the second control signal. The temperature control unit adjusts the heating film on the absorption cell according to the second control signal.

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