CN117686043A - Thermal type gas flow compensation calculation method and thermal type gas flow sensor - Google Patents
Thermal type gas flow compensation calculation method and thermal type gas flow sensor Download PDFInfo
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Abstract
The invention relates to the technical field of thermal type gas flow sensors, in particular to a thermal type gas flow compensation calculation method and a thermal type gas flow sensor. The invention has three improvements, namely, through the sense resistor R S1 The current fluctuation of the heating resistor is measured, and the actual gas flow is corrected by calculation, so that the measurement error caused by the current fluctuation can be remarkably reduced. Secondly, the gas flow is measured in a new current excitation mode through two heating resistors, and the junctionThe combined cross-correlation algorithm yields the flow rate of moisture through the sensor, which is converted to flow. Third, a shunt resistor R is adopted S2 Shunt resistor R S3 And the analog switch SW2 satisfy both wide range and low power consumption.
Description
Technical Field
The invention relates to the technical field of thermal type gas flow sensors, in particular to a thermal type gas flow compensation calculation method and a thermal type gas flow sensor.
Background
With the continuous development of technology, measurement and control of gas flow plays an increasingly important role in various fields, such as environmental monitoring, medical devices, chemical production, and the like. To meet the demands of these applications for accuracy, stability and reliability of gas flow measurements, thermal gas mass flow sensors based on MEMS (microelectromechanical systems) technology have been developed.
The MEMS thermal gas mass flow sensor is a sensor that measures gas flow using a micro-heater and a temperature sensitive element. The working principle is that the gas is heated by a heater, and then the temperature change of the gas in a heating area is detected by a temperature sensitive element, so that the mass flow of the gas is calculated. Because the MEMS technology has the advantages of miniaturization, integration, low power consumption, high performance and the like, the MEMS thermal type gas mass flow sensor has wide application prospect in the field of gas flow measurement.
The defects are that: 1. as can be seen from the working principle of the gas flow measuring sensor based on the thermopile principle, the system generally applies a constant voltage or a constant current to two ends of a heating resistor, but the traditional constant current source or constant voltage source circuit has certain fluctuation, which directly causes that the output voltage signal and the flow change cannot meet the linear relation, and particularly, the gas in the pipe body is in a tiny flow range. 2. There is occasional mixing of other gases in the measured gas, particularly substances with a significant difference in heat transfer coefficient from air, which directly causes the measured signal to be distorted at the same flow rate. 3. Thermal flow sensing systems may be applied in some low power scenarios, for example: a gas meter. However, to increase the range, it is conventional practice to increase the excitation current, but this undoubtedly increases the power consumption for battery powered products.
Disclosure of Invention
The invention provides a thermal type gas flow compensation calculation method, which comprises the following steps: adding a heating unit R L Sense resistor R connected in series S1 ；
Through sense resistor R S1 Acquiring the actual current of the heating unit;
when the power supply of the heating unit fluctuates, the error caused by the fluctuation of the power supply is compensated, and the actual gas flow is calculated, wherein the specific formula is as follows:
in which Q air For the actual gas flow, U out.m For the loop output voltage value at power supply fluctuation, I RL，m The actual current of the heating resistor when the power supply fluctuates; k is the calibration coefficient of the flow converted from the signal of the thermal gas flow sensor.
The embodiment has the advantages that the signal fluctuation caused by the current fluctuation of the heating resistor of the traditional thermopile causes the reduction of the final flow precision and repeatability, and the invention passes through the sense resistor R S1 The current fluctuation of the heating resistor is measured, and the actual gas flow is corrected by calculation, so that the measurement error caused by the current fluctuation can be remarkably reduced.
Preferably, the heating resistor R L Comprising a first resistor J 1 And a second resistance J 2 First resistor J 1 And a second resistor J 2 The interval is W;
when the first resistor J 1 When in power-on heating, the second resistor J 2 Not powering on; when the second resistor J 2 When in power-on heating, the first resistor J 1 Not powering on;
the first resistors J are alternately used as the fixed power-on time length 1 And a second resistance J 2 Electrifying to obtain a first resistor J 1 And a second resistance J 2 Up-down current voltage U when power is on D1 、U D2 、U D3 And U D4 ；
By the formulaCalculating the transit time tau, which is R (τ) A value corresponding to the maximum;
according to the formulaCalculating the gas flow rate;
the gas flow rate is calculated by multiplying the gas flow rate by the sectional area of the flow channel.
An advantage of this embodiment is that conventional thermopile thermal sensors may suffer from inaccurate measurements due to the mixing of magazines or moisture in the gas being measured; according to the invention, the gas flow is measured in a new current excitation mode through the two heating resistors, the flow rate of moisture passing through the sensor is obtained by combining a cross correlation algorithm, and the flow rate is converted into the flow rate.
In another aspect, the present invention provides a thermal gas flow sensor, comprising: the device comprises a constant current source/constant voltage source, a thermopile sensing circuit, a potential difference measuring circuit and an analog-to-digital conversion ADC;
the constant current source/constant voltage source provides working power for the thermopile sensing circuit through the current control and switching circuit, the potential difference measuring circuit extracts the potential difference of the thermopile sensor, the potential difference is sent to the MCU after passing through the analog-to-digital ADC, and the MCU calculates the gas flow according to the potential difference.
As a preferable aspect of the above-described technical solution, the constant current source/constant voltage source includes a DC-DC power source;
the thermopile sensing circuit comprises a heating unit R L And sense resistor R S1 ；
The potential difference measuring circuit comprises a differential amplifying circuit OP1 and a low-pass filter circuit LP1;
the DC-DC power supply output end passes through the heating unit R L And sense resistor R S1 Grounding; heating unit R L Sense resistor R S Is used as the input of the differential amplifying circuit OP1, the output of the differential amplifying circuit OP1 is connected with the input of the low-pass filter circuit LP1, and the output of the low-pass filter circuit LP1 passes through digital-analogThe converter ADC1 is connected with the MCU;
the MCU compensates the gas flow measurement error caused by power supply fluctuation according to the received information.
Preferably, the heating unit R is L Comprising a first resistor J 1 And a second resistance J 2 The method comprises the steps of carrying out a first treatment on the surface of the The differential amplifying circuit OP1 includes a first differential amplifying sub-circuit INA1 and a second differential amplifying sub-circuit INA2;
first resistor J 1 The up-down current voltages D1 and D2 are connected with the input end of the first differential amplifying sub-circuit INA1, the second resistor J 2 The up-down current voltages D3 and D4 are connected with the input end of the second differential amplifying sub-circuit INA2;
the output end of the first differential amplification sub-circuit INA1 is connected with the state 1 end of the analog switch SW1, the output end of the second differential amplification sub-circuit INA2 is connected with the state 2 end of the analog switch SW1, and the analog switch SW1 is connected with the singlechip MCU through a low-pass filter circuit and a digital-to-analog converter;
the sensor further comprises an excitation current/voltage measurement circuit and a current control switching circuit;
excitation current/voltage measuring circuit for calibrating first resistor J 1 And a second resistance J 2 The energizing time, the exciting current/voltage measuring circuit is connected with the MCU through the analog-to-digital conversion ADC;
and the current control switching circuit receives the MCU control instruction and controls the analog switch SW1.
Preferably, the sensor further comprises a shunt resistor R S2 And shunt resistor R S3 ；
The feedback end of the DC-DC power supply passes through a shunt resistor R S3 Connected to the heating unit R L And sense resistor R S1 Between them; feedback end of DC-DC power supply and shunt resistor R S3 Between through shunt resistor R S2 And an analog switch SW2 is connected to ground.
The advantage of this embodiment is that the conventional thermopile gas sensor uses a single current for excitation measurement, if a larger current is used, although a small flow signalThe quality is improved, but the power consumption is larger, and if smaller current is adopted, the flow range cannot be satisfied; the invention adopts a shunt resistor R S2 Shunt resistor R S3 And the analog switch SW2 satisfy both wide range and low power consumption.
The foregoing description is only an overview of the present invention, and is intended to be implemented in accordance with the teachings of the present invention in order that the same may be more clearly understood and to make the same and other objects, features and advantages of the present invention more readily apparent.
Drawings
FIG. 1 is a schematic block diagram of a thermal gas flow sensor of example 1;
fig. 2 is a schematic circuit configuration diagram of a thermopile sensor in embodiment 1;
FIG. 3 is a schematic diagram of the structure of a thermopile sensing circuit in embodiment 2;
FIG. 4 is a graph showing the current signals applied to the first resistor and the second resistor in example 2;
fig. 5 is an induced electromotive force signal measurement amplifying circuit of embodiment 2;
fig. 6 is a schematic circuit configuration diagram of a thermopile sensor in embodiment 3.
Detailed Description
In order to make the objects, features and advantages of the present invention more comprehensible, the technical solutions according to the embodiments of the present invention will be clearly described in the following with reference to the accompanying drawings, and it is obvious that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Example 1:
a thermal type gas flow compensation calculation method for eliminating current fluctuation comprises the following steps:
adding a heating unit R L Sense resistor R connected in series S1 ；
Through sense resistor R S1 Acquiring the actual current of the heating unit;
when the power supply of the heating unit fluctuates, the error caused by the fluctuation of the power supply is compensated, and the actual gas flow is calculated, wherein the specific formula is as follows:
in which Q air For the actual gas flow, U out.m For the loop output voltage value at power supply fluctuation, I RL，m The actual current of the heating resistor when the power supply fluctuates; k is the calibration coefficient of the flow converted from the signal of the thermal gas flow sensor.
The circuit structure of the thermal gas flow sensor for eliminating current fluctuation is shown in figure 1, and comprises a constant current source/constant voltage source, a thermopile sensing circuit, a potential difference measuring circuit and an analog-to-digital conversion ADC;
the constant current source/constant voltage source comprises a DC-DC power supply; the thermopile sensing circuit comprises a heating unit R L And sense resistor R S1 The method comprises the steps of carrying out a first treatment on the surface of the The potential difference measuring circuit comprises a differential amplifying circuit OP1 and a low-pass filter circuit LP1; the specific connection mode is shown in figure 2, and the output end of the DC-DC power supply passes through the heating unit R L And sense resistor R S1 Grounding; heating unit R L Sense resistor R S The voltage of the low-pass filter circuit LP1 is used as the input of the differential amplifying circuit OP1, the output of the differential amplifying circuit OP1 is connected with the input of the low-pass filter circuit LP1, and the output of the low-pass filter circuit LP1 is connected with the singlechip MCU through the digital-to-analog converter ADC 1; the MCU compensates the gas flow measurement error caused by power supply fluctuation according to the received information.
In this embodiment, the thermal flow calculation is based on:
Q fc ＝h fc A(T u -T f )，h fc ＝λNu/L
where λ is the air thermal conductivity, L is the characteristic dimension of the thermal resistor, nu is the noose number, a represents the heat exchange surface area between the air flow and the heating resistor, and Tu and Tf represent the temperature of the heating resistor and the air flow, respectively. The relation between the gas mass flow and the heating resistance can be derived by the formula:
wherein D is the characteristic length of the flow channel, mu is the aerodynamic viscosity of air, and P r Is the average plurter number and n is a constant related to the physical state of the fluid.
The relationship between the voltage signal and the upstream and downstream temperature differences is as follows:
U up ＝Na(T up -T cold )，U down ＝Na(T down -T cold )，U out ＝U down -U up ＝Na(T down -T up )＝NaΔT
wherein U is up ，U down ，U out Representing the output voltage values of the upper and lower streams of the thermopile and the loop, T up ，T doum ，T cold The temperature of the upper and lower thermal junctions of the thermopile and the temperature of the cold junction of the thermopile are respectively, delta T represents the temperature difference value of the thermal junctions of the upper and lower thermal junctions, N is the number of thermocouples forming the thermopile, and a is the Seebeck coefficient of the thermocouples.
The working principle of the thermal gas flow sensor is as follows:
the constant current source or the constant voltage source clamps the current or the voltage to the heating resistor R L And sense resistor R S The temperature drift of the sensing resistor is required to be below 0.5ppm, and the resistance value of the sensing resistor is required to be 0.1 ohm. Due to the heating resistance R L And sense resistor R S In a series relationship, so that a current I flows through the heating resistor RL And current I of sense resistor RS Equal by measuring R S Voltage value at U RS Through a differential amplifying circuit OP1 and a low-pass filter LP1, the output voltage Uo1 is acquired by a digital-to-analog converter ADC1 and is input into a singlechip MCU, the power of a heating resistor is known to be P=UI according to the thermopile principle, the change of U or I can be seen, the temperature generated by the heating resistor also changes, and the temperature is calculated from heatSignal U of electric pile out ＝U down -U up ＝Na(T down -T up ) As can be seen from =na Δt, Δt is affected. To eliminate the influence of constant voltage source fluctuation, ADC1 acquires signal Uo1 divided by sense resistor R S I.e. the current I of the heating resistor RL . Therefore, eliminating the influence of the constant voltage source voltage fluctuation can be according to the formulaThus there isWherein DeltaT i For initial temperature difference, deltaT m Temperature difference at any stage, I RL，i For the initial heating resistance current value, I RL，m The heating resistance current value measured at any stage. According to the above formula, when the current changes due to the change of the resistance of the heating resistor or other reasons, the compensated thermopile signal can be obtained according to the formulaObtained. When the system adopts the constant current source, if the flow result fluctuation caused by the current fluctuation is solved by the following way, the flow result fluctuation can be directly solved by the following way>Where K is the scaling factor for the conversion of the thermopile signal into flow.
Example 2:
a thermal gas flow compensation calculation method for eliminating moisture errors comprises the following steps: the heating resistor R L Comprising a first resistor J 1 And a second resistance J 2 First resistor J 1 And a second resistor J 2 The interval is W;
when the first resistor J 1 When in power-on heating, the second resistor J 2 Not powering on; when the second resistor J 2 When in power-on heating, the first resistor J 1 Not powering on;
the first resistors J are alternately used as the fixed power-on time length 1 And a second resistance J 2 Electrifying to obtain a first resistor J 1 And a second resistance J 2 Up-down current voltage U when power is on D1 、U D2 、U D3 And U D4 ；
By the formulaCalculating the transit time tau, which is R (τ) A value corresponding to the maximum;
according to the formulaCalculating the gas flow rate;
the gas flow rate is calculated by multiplying the gas flow rate by the sectional area of the flow channel.
Heating unit R for realizing elimination of moisture error and thermal type gas flow sensor L Comprising a first resistor J 1 And a second resistance J 2 The method comprises the steps of carrying out a first treatment on the surface of the The differential amplifying circuit OP1 includes a first differential amplifying sub-circuit INA1 and a second differential amplifying sub-circuit INA2;
first resistor J 1 The up-down current voltages D1 and D2 are connected with the input end of the first differential amplifying sub-circuit INA1, the second resistor J 2 The up-down current voltages D3 and D4 are connected with the input end of the second differential amplifying sub-circuit INA2;
the output end of the first differential amplification sub-circuit INA1 is connected with the state 1 end of the analog switch SW1, the output end of the second differential amplification sub-circuit INA2 is connected with the state 2 end of the analog switch SW1, and the analog switch SW1 is connected with the singlechip MCU through a low-pass filter circuit and a digital-to-analog converter;
the sensor further comprises an excitation current/voltage measurement circuit and a current control switching circuit;
excitation current/voltage measuring circuit for calibrating first resistor J 1 And a second resistance J 2 The energizing time, the exciting current/voltage measuring circuit is connected with the MCU through the analog-to-digital conversion ADC;
and the current control switching circuit receives the MCU control instruction and controls the analog switch SW1.
In this embodiment, compared with the conventional thermopile gas flow sensor, as shown in fig. 3, the thermopile sensing circuit includes two heating resistors of the same specification, and a thermopile of the same specification is disposed upstream and downstream of each heating resistor, and the distance between the thermopiles is w=8mm. Fig. 4 is a current signal on a heating resistor, fig. 5 is an induced electromotive force signal measurement amplifying circuit, and in combination with fig. 3, fig. 4 and fig. 5, the first state is that when the heating resistor J1 is electrified, the heating resistor J2 is not electrified for 100ms, signals of thermopiles D1 and D2 are sent to a differential amplifying circuit INA1, and the amplified signals are sent to a singlechip MCU through an SW1 state 1 and then through an analog-digital conversion ADC 2; the second state is that the heating resistor J1 is not electrified, the heating resistor J2 is electrified, the duration is 100ms, signals of the thermopiles D3 and D4 are sent to the differential amplifying circuit INA2, and the amplified signals are sent to the singlechip MCU through the SWl state 2 and the analog-digital conversion ADC 2. The state of SW1 is the same as the order of excitation of two heating resistors and is controlled by MCU. When the detected gas is not interfered by moisture, the flow values calculated by the signals in the two acquired states are the same. However, when moisture or other impurities are mixed into the gas to be measured, the time of the delay of the signals in two states can be calculated by cross-correlation, and the specific formula is as follows:
where the value of the transit time τ is when R (τ) is the maximum value, and because the distance between the two sets of thermopiles is a known amount of 8mm, its speed can be determined as:
the flow rate multiplied by the cross-sectional area of the flow channel is equal to the flow rate.
Example 3:
thermal type gas flow sensor capable of reducing power consumption and newly added shunt resistor R S2 And shunt resistor R S3 The feedback end of the DC-DC power supply passes through a shunt resistor R S3 Connected to the heating unit R L And sense resistor R S1 Between them; feedback end of DC-DC power supply and shunt resistor R S3 Between through shunt resistor R S2 And an analog switch SW2 is connected to ground.
In this embodiment, the essence of the improvement of the measuring range is to increase the signal-to-noise ratio, and the current value flowing through the heating resistor is usually increased, but if a large current is used for a long time or under the condition of a large flow, the power consumption is increased, so the invention proposes a scheme for replacing the traditional thermopile power supply. As shown in fig. 6, the specific steps are as follows:
in the initial state, the DC-DC voltage source circuit outputs a voltage to the sampling resistor R s1 On the contrary, a certain IO port of the MCU singlechip outputs a control signal to the analog switch SW2, at the moment, the analog switch SW2 is in an open state, and the output current is I s1 In this state, the sensor mainly works when the flow is large; when the flow rate is judged to enter a smaller flow rate range by the MCU program, the MCU outputs a control signal to close the analog switch SW2 through the same IO port, and the output current is I s2 Due to R s2 And R is s3 Relation of partial pressure, I s2 ＞I s1 Thereby realizing a large current output. In addition, when the pipeline enters an empty pipe state, a control signal is output through the other IO port of the MCU to close the DC-DC voltage source, so that the power consumption is reduced. After 10 seconds, the DC-DC voltage source was turned on again, and a measurement was made to determine whether gas was passing through the sensor.
In the description of the present specification, a description referring to terms "one embodiment," "some embodiments," "examples," "specific examples," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present invention. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, the different embodiments or examples described in this specification and the features of the different embodiments or examples may be combined and combined by those skilled in the art without contradiction.
Furthermore, the terms "first," "second," and the like, are used for descriptive purposes only and are not to be construed as indicating or implying a relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include at least one such feature. In the description of the present invention, the meaning of "a plurality" is two or more, unless explicitly defined otherwise.
The foregoing is merely illustrative of the present invention, and the present invention is not limited thereto, and any person skilled in the art will readily recognize that variations or substitutions are within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (6)
1. A thermal type gas flow compensation calculation method is characterized by comprising the following specific steps: adding a heating unit R L Sense resistor R connected in series S1 ；
Through sense resistor R S1 Acquiring the actual current of the heating unit;
when the power supply of the heating unit fluctuates, the error caused by the fluctuation of the power supply is compensated, and the actual gas flow is calculated, wherein the specific formula is as follows:
in which Q air For the actual gas flow, U out.m For the loop output voltage value at power supply fluctuation, I RL，m The actual current of the heating resistor when the power supply fluctuates; k is the calibration coefficient of the flow converted from the signal of the thermal gas flow sensor.
2. The method according to claim 1, wherein the heating resistor R L Comprising a first electricityResistance J 1 And a second resistance J 2 First resistor J 1 And a second resistor J 2 The interval is W;
when the first resistor J 1 When in power-on heating, the second resistor J 2 Not powering on; when the second resistor J 2 When in power-on heating, the first resistor J 1 Not powering on;
the first resistors J are alternately used as the fixed power-on time length 1 And a second resistance J 2 Electrifying to obtain a first resistor J 1 And a second resistance J 2 Up-down current voltage U when power is on D1 、U D2 、U D3 And U D4 ；
By the formulaCalculating the transit time tau, which is R (τ) A value corresponding to the maximum;
according to the formulaCalculating the gas flow rate;
the gas flow rate is calculated by multiplying the gas flow rate by the sectional area of the flow channel.
3. A thermal gas flow sensor, comprising: the device comprises a constant current source/constant voltage source, a thermopile sensing circuit, a potential difference measuring circuit and an analog-to-digital conversion ADC;
the constant current source/constant voltage source provides working power for the thermopile sensing circuit through the current control and switching circuit, the potential difference measuring circuit extracts the potential difference of the thermopile sensor, the potential difference is sent to the MCU after passing through the analog-to-digital ADC, and the MCU calculates the gas flow according to the potential difference.
4. A thermal gas flow sensor according to claim 3, wherein the constant current/constant voltage source comprises a DC-DC power supply;
the thermopile sensing circuit comprises a heating unit R L And sense resistor R S1 ；
The potential difference measuring circuit comprises a differential amplifying circuit OP1 and a low-pass filter circuit LP1;
the DC-DC power supply output end passes through the heating unit R L And sense resistor R S1 Grounding; heating unit R L Sense resistor R S The voltage of the low-pass filter circuit LP1 is used as the input of the differential amplifying circuit OP1, the output of the differential amplifying circuit OP1 is connected with the input of the low-pass filter circuit LP1, and the output of the low-pass filter circuit LP1 is connected with the singlechip MCU through the digital-to-analog converter ADC 1;
the MCU compensates the gas flow measurement error caused by power supply fluctuation according to the received information.
5. The thermal gas flow sensor according to claim 4, wherein the heating unit R L Comprising a first resistor J 1 And a second resistance J 2 The method comprises the steps of carrying out a first treatment on the surface of the The differential amplifying circuit OP1 includes a first differential amplifying sub-circuit INA1 and a second differential amplifying sub-circuit INA2;
the first resistor J1 up-down current voltages D1 and D2 are connected with the input end of the first differential amplifier sub-circuit INA1, and the second resistor J2 up-down current voltages D3 and D4 are connected with the input end of the second differential amplifier sub-circuit INA2;
the output end of the first differential amplification sub-circuit INA1 is connected with the state 1 end of the analog switch SW1, the output end of the second differential amplification sub-circuit INA2 is connected with the state 2 end of the analog switch SW1, and the analog switch SW1 is connected with the singlechip MCU through a low-pass filter circuit and a digital-to-analog converter;
the sensor further comprises an excitation current/voltage measurement circuit and a current control switching circuit;
excitation current/voltage measuring circuit for calibrating first resistor J 1 And a second resistance J 2 The energizing time, the exciting current/voltage measuring circuit is connected with the MCU through the analog-to-digital conversion ADC;
and the current control switching circuit receives the MCU control instruction and controls the analog switch SW1.
6. The thermal gas stream of claim 5The quantity sensor is characterized in that the sensor also comprises a shunt resistor R S2 And shunt resistor R S3 ；
The feedback end of the DC-DC power supply passes through a shunt resistor R S3 Connected to the heating unit R L And sense resistor R S1 Between them; feedback end of DC-DC power supply and shunt resistor R S3 Between through shunt resistor R S2 And an analog switch SW2 is connected to ground.
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