Automatic temperature compensation circuit and method based on thermal flowmeter

Abstract

The invention discloses an automatic temperature compensation circuit and method based on a thermal flowmeter, and relates to the technical field of temperature compensation, and the compensation circuit comprises a constant current source module used for driving the thermal flowmeter; the Wheatstone virtual bridge arm is used for acquiring a changed digital-to-analog converter output voltage value output by the controller, and performing voltage conversion on a first voltage and a second voltage which are output on the thermal flowmeter due to the change of the external temperature according to the output voltage value to obtain a third voltage; the digital-to-analog converter is used for performing correction compensation on the third voltage according to the correction compensation value calculated by the controller; and the differential amplification module adopts a differential amplifier to amplify the third voltage and the fourth voltage output by the thermal flowmeter, and outputs a compensated flow signal. The compensating circuit is simple in structure and higher in reliability and precision, and compared with a digital potentiometer, the output value of the Wheatstone virtual bridge arm is changed by using the output value of the digital-to-analog converter, so that the adjusting resolution is higher.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of temperature compensation, in particular to an automatic temperature compensation circuit and method based on a thermal flowmeter. BACKGROUND

[0002] A non-contact thermal gas mass flowmeter (hereinafter referred to as a thermal flowmeter) has two or three groups of temperature-sensitive resistance wires wound on a capillary pipe in the flow sensor. By heating the resistance wire groups, the resistance wire groups maintain a certain temperature. When fluid passes through, the thermal balance between the resistance wire groups is broken, and the heat of the upstream resistance wire is transferred to the downstream resistance wire with the fluid, causing the resistance difference between the resistance wire groups to change. By measuring the change in resistance between the resistance wire groups, the corresponding change can be obtained, and the flow through the capillary pipe can be obtained, and the fluid flow through the thermal flowmeter can be obtained.

[0003] Since the thermal flowmeter obtains the current flow value by measuring the temperature change between the internal resistance wires of the sensor, environmental temperature and fluid temperature and other factors will greatly affect the measurement accuracy of the thermal flowmeter. For example, when the ambient temperature is 23 degrees Celsius, the thermal flowmeter shows zero flow when there is no flow, but when the ambient temperature is 35 degrees Celsius, the thermal flowmeter shows a flow of 5 SCCM when there is no flow. The reason is that due to the change in external temperature, the resistance of the resistance wire wound on the capillary pipe shifts, changing the thermal balance inside the flow sensor.

[0004] There are usually two methods to reduce the influence of external temperature change on the thermal flowmeter. The first method is to compensate through software. The external ambient temperature is measured by a temperature sensor, and a function of the external ambient and flow value change is established , and the flow value is compensated by the function . However, since the flow sensor of the thermal flowmeter is difficult to unify, the temperature compensation function of each flowmeter is different. If the temperature experiment is performed on each flowmeter to obtain the function , it will be very cumbersome and difficult to compensate. The second method is to compensate through hardware, as shown in Figure 1 . The principle is to use a temperature drift resistance with the same characteristics to act as one bridge arm of a Wheatstone bridge. When the external temperature changes, both the left and right bridge arms of the Wheatstone bridge change, thereby reducing the influence of the external temperature on the flowmeter. However, such temperature drift resistance is usually difficult to find, and the debugging is difficult. The program does not know the true state of the current flow sensor, and cannot compensate it through software.

[0005] An automatic temperature compensation circuit and method for a thermal flow meter, as disclosed in patent publication number CN117824771A, is shown. Figure 2 As shown, it uses a digital potentiometer to correct the flow sensor signal. When the ambient temperature changes, the program adjusts the potentiometer's setting to balance the Wheatstone bridge, thus achieving automatic temperature compensation. However, since most common digital potentiometers have only 256-bit and 1024-bit adjustment settings, their resolution is low. Furthermore, when the temperature changes, the program cannot directly determine how much the current potentiometer setting should be increased or decreased; therefore, prior experimentation is needed to determine the appropriate function. Subsequently, according to the function The compensation method for adjusting the digital potentiometer is not very flexible. Summary of the Invention

[0006] The purpose of this invention is to provide an automatic temperature compensation circuit and method based on a thermal flow meter, which solves the problems of low resolution, complex adjustment methods, and inflexible compensation methods in the prior art.

[0007] This invention is achieved through the following technical solution: In a first aspect, the first embodiment of the present invention provides an automatic temperature compensation circuit based on a thermal flow meter, comprising: a constant current source module, a Wheatstone virtual bridge arm, a digital-to-analog converter, and a differential amplifier module; The constant current source module is used to drive the thermal flow meter; The Wheatstone virtual bridge arm is used to obtain the changed digital-to-analog converter output voltage value from the controller, and to perform voltage conversion based on the output voltage value, the first voltage and the second voltage output by the thermal flow meter due to changes in the external temperature, to obtain the third voltage. The digital-to-analog converter is used to correct and compensate the third voltage according to the correction compensation value calculated by the controller; The differential amplifier module uses a differential amplifier to amplify the third voltage and the fourth voltage output by the thermal flow meter, and outputs a compensated flow signal.

[0008] Furthermore, the Wheatstone virtual bridge arm includes a third amplifier, a fourth resistor, a fifth resistor, a seventh resistor, and a twelfth resistor, with the fifth, seventh, and twelfth resistors having the same resistance value.

[0009] Furthermore, the calculation formula for voltage conversion based on the output voltage value, the first voltage and the second voltage output by the thermal flow meter due to changes in external temperature is as follows: ; in, The voltage at point V1, The voltage at point V2, The output voltage value of the digital-to-analog converter, The voltage of the V3 point is the output value of the Wheatstone virtual bridge arm.

[0010] Further, the positive input end of the differential amplification module is connected with a third voltage, the negative input end is connected with a fourth voltage, the two gain ends are connected through a resistor, the positive power supply end is connected with a power supply VCC, and the negative power supply end is connected with a power supply-VCC, and the output end of the differential amplification module outputs a compensated flow signal.

[0011] Further, the differential amplification module adopts an AD8226 instrument amplifier.

[0012] Further, the digital-to-analog converter adopts a digital-to-analog conversion chip model TPC116S1-VR.

[0013] In a second aspect, another embodiment of the present application provides an automatic temperature compensation method based on a thermal flowmeter, which is suitable for the automatic temperature compensation circuit based on the thermal flowmeter described in the first embodiment, and the method comprises the following steps: Obtaining a flow signal collected by the thermal flowmeter; Processing the flow signal through the automatic temperature compensation circuit based on the thermal flowmeter to obtain a processed flow signal; Judging whether to perform zero-point automatic zero compensation or flow temperature automatic compensation on the flow signal according to the processed flow signal to obtain a correction compensation value; Transmitting the correction compensation value to the digital-to-analog converter; Controlling the digital-to-analog converter to correct and compensate the output value of the Wheatstone virtual bridge arm according to the correction compensation value, so that the two input voltage values of the differential amplification module are equal, and the differential amplification module outputs a compensated flow signal.

[0014] Further, the specific method for judging whether to perform zero-point automatic zero compensation on the flow signal according to the processed flow signal comprises the following steps: Obtaining a zero-point flow when the valves of the thermal flowmeter are all closed; Judging whether the voltage value output by the differential amplification module is zero; If not, calculating a fifth voltage output by the current digital-to-analog converter according to the values of the first voltage, the second voltage and the fourth voltage to ensure that the two input voltage values of the differential amplification module are equal; If yes, maintaining the output voltage value of the current digital-to-analog converter; Controlling the thermal flowmeter to display zero flow.

[0015] Further, the calculation formula for calculating the output voltage value of the current digital-to-analog converter according to the values of the first voltage, the second voltage and the fourth voltage is as follows: ; wherein, V1 is the voltage of the point V1, V2 is the voltage of the point V2, V3 is the output voltage value of the digital-to-analog converter, V4 is the voltage of the point V4.

[0016] Further, the specific method for automatically compensating the flow temperature when adjusting the flow signal according to the processed flow signal comprises the following steps: obtaining a relationship function between the change of the ambient temperature and the flow output signal during adjustment according to the temperature experiment in advance; when the external temperature changes, obtaining the change value of the external temperature collected by the temperature sensor, and obtaining the output voltage change value of the thermal flowmeter after being amplified by the differential amplification module according to the relationship function; according to the voltage change value and the voltage amplification multiple of the differential amplification module, calculating the corresponding change of the output voltage value of the digital-to-analog converter; correcting and compensating the output value of the Wheatstone virtual bridge arm according to the corresponding change of the voltage value.

[0017] Compared with the prior art, the present application has the following advantages and beneficial effects: The automatic temperature compensation circuit based on the thermal flowmeter provided by the embodiment of the present application has a simple structure, higher reliability and precision, and compared with the digital potentiometer, the use of the output value of the digital-to-analog converter to change the output value of the Wheatstone virtual bridge arm has higher adjustment resolution.

[0018] The automatic temperature compensation method based on the thermal flowmeter provided by the embodiment of the present application can automatically compensate the zero flow value and the flow value during adjustment of the thermal flowmeter, has higher flexibility and reliability compared with the traditional compensation method, and the use of the output value of the digital-to-analog converter to change the output value of the Wheatstone virtual bridge arm has higher adjustment resolution. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be considered as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor. In the drawings: Figure 1 is a traditional hardware compensation circuit diagram; Figure 2 is a circuit diagram of the automatic temperature compensation circuit for the thermal flowmeter disclosed in the patent application with publication number CN117824771A; Figure 3 A principle block diagram of an automatic temperature compensation circuit based on a thermal flowmeter provided for the first embodiment of the present application; Figure 4 A circuit diagram of an automatic temperature compensation circuit based on a thermal flowmeter provided for the first embodiment of the present application; Figure 5 A flow chart of an automatic temperature compensation method based on a thermal flowmeter provided for another embodiment of the present application; Figure 6 A flow chart of a zero-point automatic zeroing compensation method in another embodiment of the present application; Figure 7 A flow chart of a flow temperature automatic compensation during adjustment in another embodiment of the present application. DETAILED DESCRIPTION

[0020] In order to make the objects, technical solutions and advantages of the present application clearer, further detailed description will be made to the present application in combination with embodiments and drawings, the illustrative embodiments and their description are only used to explain the present application, and do not limit the present application.

[0021] As Figure 3 , 4As shown, the first embodiment of the present application provides an automatic temperature compensation circuit based on a thermal flowmeter, which comprises a constant current source module, a Wheatstone virtual bridge arm, a digital-to-analog converter and a differential amplification module. The constant current source module is used to drive the flow sensor and comprises a fourth amplifier U4, a first resistor R1, a second resistor R2, a third resistor R3, an eighth resistor R8 and a MOS tube Q1. The MOS tube Q1 is an N-channel MOS tube. The third resistor R3 and the eighth resistor R8 are two groups of resistance wires on the thermal flowmeter, serving as one bridge arm of the Wheatstone bridge. The positive power supply end of the fourth amplifier U4 is connected to a power supply VCC, the negative power supply end is connected to a power supply-VCC, the positive input end is connected to a VREF end, the negative input end is connected to one end of the second resistor R2, the other end of the second resistor R2 is grounded, the output end of the fourth amplifier U4 is connected to the gate of the MOS tube Q1, the drain of the MOS tube Q1 is connected to one end of the first resistor R1, the other end of the first resistor R1 is connected to the power supply VCC, the source of the MOS tube Q1 is connected to one end of the third resistor R3 and connected to an ADC1 of a controller and a first voltage end V1, the other end of the third resistor R3 is connected to one end of the eighth resistor R8 and an ADC3 of the controller, the ADC3 end is connected to one end of a sixth resistor in the differential amplification module, and the other end of the eighth resistor R8 is connected to an ADC2 of the controller and a second voltage end V2. The Wheatstone virtual bridge arm serves as another bridge arm of the Wheatstone bridge and comprises a third amplifier U3, a fourth resistor R4, a fifth resistor R5, a seventh resistor R7 and a twelfth resistor R12. The fifth resistor R5, the seventh resistor R7 and the twelfth resistor R12 have equal resistance values and are used for voltage conversion. The calculation formula of the voltage conversion is: ; wherein, V1 is the voltage of a point, V2 is the voltage of a point, Vout is the output voltage value of the digital-to-analog converter, V3 is the voltage of a point, and is the output value of the Wheatstone virtual bridge arm.

[0022] The digital-to-analog converter U2 compensates the voltage V3 of the Wheatstone virtual bridge arm part. The digital-to-analog converter U2 adopts a digital-to-analog conversion chip model TPC116S1-VR. The differential amplification module is used for amplifying the signal output by the thermal flowmeter and comprises a first amplifier U1, a sixth resistor R6, a ninth resistor R9, a tenth resistor R10 and an eleventh resistor R11. The positive input end of the differential amplification module is connected to a third voltage V3, the negative input end is connected to a fourth voltage V4, the two gain ends are connected through the ninth resistor R9, the positive power supply end is connected to a power supply VCC, the negative power supply end is connected to a power supply-VCC, the reference end is grounded, and the output end of the differential amplification module outputs the compensated flow signal through the tenth resistor R10. The U1 selects an AD8226 instrument amplifier, and the calculation formula of the amplification multiple G is: ; wherein, is R9.

[0023] AD8226 is a low-cost, wide power range instrument amplifier, which can set any gain between 1 to 1000 by only one external resistor, and realize the adjustment of amplification multiple, and its output signal , the calculation formula is as follows:

[0024] wherein, and are the voltages of V3 point and V4 point respectively, is the reference voltage of the digital-to-analog converter U2.

[0025] When the external temperature changes, due to the temperature drift of the third resistor R3 and the eighth resistor R8 on the thermal flowmeter, the fourth voltage V4 changes, at this time, the controller changes the output value of the digital-to-analog converter U2, and the voltage operation is carried out through the Wheatstone virtual bridge arm, and then the third voltage V3 is changed, so that the output voltage value of the differential amplification module remains unchanged. Compared with the traditional hardware temperature compensation, the structure of the compensation circuit is simple, the reliability and precision are higher, and compared with the digital potentiometer, the use of the digital-to-analog converter output value to change the output value of the Wheatstone virtual bridge arm has higher adjustment resolution, the adjustment method is simple, and the flexibility is high.

[0026] As shown in Figure 5 , another embodiment of the present application provides an automatic temperature compensation method based on a thermal flowmeter, which is suitable for the automatic temperature compensation circuit based on the thermal flowmeter described in the above embodiment, and the method comprises the following steps: acquiring a flow signal collected by the thermal flowmeter; processing the flow signal through the automatic temperature compensation circuit based on the thermal flowmeter to obtain a processed flow signal; judging whether to perform zero-point automatic zero compensation or flow temperature automatic compensation on the flow signal according to the processed flow signal to obtain a correction compensation value; transmitting the correction compensation value to the digital-to-analog converter; controlling the data converter to correct and compensate the output value of the Wheatstone virtual bridge arm according to the correction compensation value, so that the two input voltage values of the differential amplification module are equal, and the differential amplification module outputs the compensated flow signal.

[0027] As shown in Figure 6 , the specific method for judging whether to perform zero-point automatic zero compensation on the flow signal according to the processed flow signal comprises: Acquiring the valve of the thermal flowmeter is all closed state, acquiring zero point flow; Judging whether the voltage value output by the differential amplification module is zero or not; If not, calculating the fifth voltage output by the current digital-to-analog converter according to the values of the first voltage, the second voltage and the fourth voltage, so as to ensure that the voltage values of the two input terminals of the differential amplification module are equal; If yes, keeping the output voltage value of the current digital-to-analog converter; Controlling the thermal flowmeter to display zero flow.

[0028] The zero automatic zero compensation method specifically comprises the following steps: zero setting of the flowmeter at a standard temperature, changing the output value fifth voltage V5 of the digital-to-analog converter U2 to make the output value of the differential amplification module U1 zero, i.e. making the voltage values of the two points V3 and V4 equal by adjusting the output value of the digital-to-analog converter U2, collecting the output voltage value of the differential amplification module U1 by the ADC module converter or the ADC of the controller, and taking the voltage value as the voltage value when the flow is zero, wherein the output value V5 of the digital-to-analog converter U2 can be determined by collecting the voltages of the points V1, V2 and V4, as shown in formula 3.

[0029] ; Wherein, V1 is the voltage of the point V1, V2 is the voltage of the point V2, V5 is the output voltage value of the digital-to-analog converter, V4 is the voltage of the point V4.

[0030] When the outside temperature changes, the voltages of the points V1, V2 and V4 change due to the temperature drift of the resistance wire of the thermal flowmeter sensor, at this time, the controller collects the voltage values of the points V1, V2 and V4, calculates the output voltage value V5 of the current digital-to-analog converter by formula, so as to ensure that the voltage values of the two points V3 and V4 are equal, and the output value of the differential amplification module U1 is still zero, i.e. the output flow value of the thermal flowmeter is still zero at this time.

[0031] As shown in Figure 7 , the specific method for automatically compensating the flow temperature when adjusting the flow signal according to the processed flow signal comprises the following steps: Acquiring a relationship function between the change of the environmental temperature and the output signal of the flow during adjustment, which is obtained according to temperature experiments in advance; When the outside temperature changes, acquiring the change value of the outside temperature collected by the temperature sensor, and acquiring the output voltage change value of the thermal flowmeter after being amplified by the differential amplification module according to the relationship function; According to the voltage change value and the voltage amplification multiple of the differential amplification module, calculating the corresponding change of the output voltage value of the digital-to-analog converter. According to the corresponding change of the voltage value, the output value of the Wheatstone virtual bridge arm is corrected and compensated.

[0032] The temperature of the flow during adjustment is automatically compensated. Firstly, a temperature experiment is conducted in combination with a temperature sensor and a thermal flowmeter compensation circuit. When the external environment temperature changes by t℃, the signal value output by the thermal flowmeter after being amplified by the differential amplification module U1 changes by xmV. The relationship function x(t) between the change of the external environment temperature and the output signal of the flow during adjustment is obtained. Secondly, according to the calculation formula of V3, when the output voltage value of the digital-to-analog converter U2 increases or decreases by 1mV, the voltage at the positive end input of the differential amplification module, i.e., V3, increases or decreases by 1 / 3mV. When the external temperature changes, the controller obtains the change value of the external temperature through the temperature sensor and obtains the change size of the output voltage value of the thermal flowmeter after being amplified by the differential amplification module U1 through the function x(t). The controller controls the output voltage value of the digital-to-analog converter U2 to change in the opposite direction by mV. mV, G is the voltage amplification multiple of the differential amplification module U1, , R9. For example, when R9=49.4KΩ, when the external temperature changes by 10℃, the signal value output by the thermal flowmeter after being amplified by the differential amplification module changes by +5mV through the function x(t). At this time, the output voltage value of the digital-to-analog converter U2 should change by-2.5mV. According to the corresponding change of the voltage value, the output value of the Wheatstone virtual bridge arm is corrected and compensated, so that the voltage values at the two points of V3 and V4 are equal.

[0033] The automatic temperature compensation method based on the thermal flowmeter provided by the embodiment of the present application can automatically compensate the zero flow value and the flow value during adjustment of the thermal flowmeter. Compared with the traditional compensation method, the present application has higher flexibility and reliability. The output value of the digital-to-analog converter changes the output value of the Wheatstone virtual bridge arm, which has higher adjustment resolution.

[0034] The above-described specific embodiments further explain the purpose, technical solutions and beneficial effects of the present application. It should be understood that the above-described specific embodiments are merely specific embodiments of the present application and are not used to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. An automatic temperature compensation circuit for a thermal flow meter, comprising: The application relates to a constant current source module, a Wheatstone virtual bridge arm, a digital-analog converter and a differential amplification module. The constant current source module is used for driving a thermal flowmeter. The Wheatstone virtual bridge arm is used for obtaining a changed digital-analog converter output voltage value of a controller output, performing voltage conversion according to the output voltage value, a first voltage and a second voltage output by the thermal flowmeter due to changes of external temperature, and obtaining a third voltage. The digital-analog converter is used for performing correction compensation on the third voltage according to a correction compensation value calculated by the controller. The differential amplification module adopts a differential amplifier to amplify the third voltage and a fourth voltage output by the thermal flowmeter, and outputs a compensated flow signal. The Wheatstone virtual bridge arm comprises a third amplifier, a fourth resistor, a fifth resistor, a seventh resistor and a twelfth resistor, and the fifth resistor, the seventh resistor and the twelfth resistor have the same resistance.

2. The automatic temperature compensation circuit based on a thermal flow meter according to claim 1, characterized in that The calculation formula of the voltage conversion according to the output voltage value, the first voltage and the second voltage output by the thermal flowmeter due to changes of external temperature is:

3. The automatic temperature compensation circuit based on a thermal flow meter of claim 1, wherein, The positive input end of the differential amplification module is connected with the third voltage, the negative input end is connected with the fourth voltage, the two gain ends are connected through resistors, the positive power supply end is connected with a power supply VCC, the negative power supply end is connected with a power supply-VCC, and the output end of the differential amplification module outputs the compensated flow signal. ； wherein, V1 is the voltage at point V1, V2 is the voltage at point V2, Vout is the output voltage value of the digital-to-analog converter, V3 is the voltage at point V3, which is the output value of the virtual Wheatstone bridge arm.

4. The automatic temperature compensation circuit based on a thermal flow meter of claim 1, wherein, The differential amplification module adopts an AD8226 instrument amplifier.

5. The automatic temperature compensation circuit based on a thermal flow meter of claim 1, wherein, The digital-analog converter adopts a digital-analog conversion chip model TPC116S1-VR.

6. The automatic temperature compensation circuit based on a thermal flow meter of claim 1, wherein, The application is suitable for an automatic temperature compensation circuit based on a thermal flowmeter, and the method comprises the following steps:

7. A method of automatic temperature compensation based on a thermal flow meter, characterized in that, acquiring a flow signal collected by the thermal flowmeter; processing the flow signal through the automatic temperature compensation circuit based on the thermal flowmeter to obtain a processed flow signal; judging whether to perform zero-point automatic zero compensation or flow temperature automatic compensation on the flow signal according to the processed flow signal, and obtaining a correction compensation value; transmitting the correction compensation value to a digital-analog converter; controlling the digital-analog converter to perform correction compensation on an output value of a Wheatstone virtual bridge arm according to the correction compensation value, so that two input voltage values of a differential amplification module are equal, and the differential amplification module outputs a compensated flow signal. The specific method for judging whether to perform zero-point automatic zero compensation on the flow signal according to the processed flow signal comprises the following steps:

8. The automatic temperature compensation method based on a thermal flow meter according to claim 7, characterized in that, acquiring zero-point flow when all valves of the thermal flowmeter are in a closed state; judging whether the voltage value output by the differential amplification module is zero; if not, calculating a fifth voltage output by the current digital-analog converter according to the values of the first voltage, the second voltage and the fourth voltage to ensure that the two input voltage values of the differential amplification module are equal; if yes, maintaining the output voltage value of the current digital-analog converter; controlling the thermal flowmeter to display zero flow. The calculation formula for calculating the output voltage value of the current digital-analog converter according to the values of the first voltage, the second voltage and the fourth voltage is:

9. The automatic temperature compensation method based on a thermal flow meter according to claim 8, characterized in that, The specific method for judging whether to perform flow temperature automatic compensation on the flow signal according to the processed flow signal comprises the following steps: ； wherein, is the voltage at point V1, is the voltage at point V2, is the output voltage value of the digital-to-analog converter, is the voltage at point V4.

10. The automatic temperature compensation method based on a thermal flow meter according to claim 7, characterized in that, acquiring a relationship function between environmental temperature changes and the flow output signal during adjustment obtained according to temperature experiments in advance; ​ When the outside temperature changes, a change value of the outside temperature collected by the temperature sensor is acquired, and a change value of the output voltage of the thermal flowmeter after being amplified by the differential amplification module is acquired according to a relationship function; According to the voltage change value and the voltage amplification multiple of the differential amplification module, a corresponding change of the output voltage value of the digital-to-analog converter is calculated; The output value of the Wheatstone virtual bridge arm is corrected and compensated according to the corresponding change of the voltage value.

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