Flow detection circuit based on nanoliter flow sensor

Abstract

The invention discloses a flow detection circuit based on a nano-liter flow sensor, which belongs to the technical field of nano-liter flow detection and comprises a flow sensor, a flow sensor driving unit, a detection unit and a power supply module. The flow sensor is connected with the flow sensor driving unit and the detection unit, and the power supply module is connected with the flow sensor driving unit and the detection unit and used for supplying power to the flow sensor driving unit and the detection unit. According to the invention, complete non-contact liquid flow detection is realized; a bidirectional flow detection function can be realized; the detection circuit is simple, small in module size, low in cost and high in detection precision; according to the design parameter difference of the flow sensor, wide-range detection can be realized; constant temperature difference control of the environment temperature is achieved in circuit design, the influence of environment temperature change on measurement is small, and an additional temperature compensation circuit is not needed.

Description

Technical Field

[0001] The present invention relates to the technical field of nanoliter flow detection, and in particular to a flow detection circuit based on a nanoliter flow sensor. Background Art

[0002] Flow detection technology is one of the key technologies in modern industry, environmental protection, detection and other fields. With the development of refined and automated resource management, the demand for nano-flow control and detection technology is increasing.

[0003] Nanoliter flow detection technology is small in size, difficult to process, and expensive. In many applications, flow detection requires contactless detection, and there are large errors in the method of inferring flow through pressure. Fluorescence detection methods will interfere with samples, and electrochemical detection methods require calibration of the relationship between dielectric constant and flow, all of which have inherent defects.

[0004] How to achieve low-cost, high-precision, and high-resolution nanoliter flow detection is an urgent problem to be solved. To this end, a flow detection circuit based on a nanoliter flow sensor is proposed. Summary of the Invention

[0005] The technical problem to be solved by the present invention is how to achieve low-cost, high-precision, and high-resolution nanoliter flow detection, and provides a flow detection circuit based on a nanoliter flow sensor.

[0006] The present invention solves the above-mentioned technical problems through the following technical solutions. The present invention includes a flow sensor, a flow sensor drive unit, a detection unit and a power supply module; the flow sensor is respectively connected to the flow sensor drive unit and the detection unit, and the power supply module is respectively connected to the flow sensor drive unit and the detection unit, and is used to power the flow sensor drive unit and the detection unit; the flow sensor is a nanoliter flow sensor, which is in contact with the liquid pipeline through a heat-conductive material and is used to detect and obtain a flow signal. The flow sensor includes an upper thermistor RU, a heating resistor RH, a lower thermistor RD and an ambient temperature detection resistor RE; the heating resistor RH and the ambient temperature detection resistor RE are connected to the flow sensor drive unit, and the flow sensor drive unit is used to drive the heating resistor RH to achieve heating; the upper thermistor RU and the lower thermistor RD are connected to the detection unit, and the detection unit is used to detect the resistance difference between the upper thermistor RU and the lower thermistor RD and convert it into a voltage difference, and then amplify and detect it.

[0007] Furthermore, in the flow sensor, heating by the heating resistor RH generates a temperature with a constant temperature difference from the environment, and the direction of liquid flow is from the upper thermistor RU to the lower thermistor RD; when the liquid is stationary, the thermal field of the heating resistor RH is uniform, and the temperatures of the upper thermistor RU and the lower thermistor RD, which are equidistant from the heating resistor RH, are the same; when the liquid flows, the thermal field of the heating resistor RH deflects toward the lower thermistor RD, and the temperature of the lower thermistor RD is higher than that of the upper thermistor RU. By detecting the resistance changes of the lower thermistor RD and the upper thermistor RU, the flow signal of the liquid can be calculated.

[0008] Furthermore, in the flow sensor, the upper thermistor RU and the lower thermistor RD have equal resistance values and are positioned symmetrically with respect to the heating resistor RH.

[0009] Furthermore, in the flow sensor, the ambient temperature detection resistor RE is used to detect the ambient temperature, and the resistance value of the heating resistor RH (as a bridge resistor) in the H-bridge is adjusted in the flow sensor drive unit to achieve the setting of the temperature difference between the two resistor positions of the ambient temperature detection resistor RE and the heating resistor RH, and the constant temperature difference control of the heating resistor RH position is achieved through the closed-loop control of the flow sensor drive unit.

[0010] Furthermore, the flow sensor drive unit includes a control tube Q1, resistors R1, R2, and R3, an operational amplifier U1, and a capacitor C1; wherein, the resistor R1 and the ambient temperature detection resistor RE are connected in series to form a first left bridge arm, the resistor R2 and the heating resistor RH are connected in series to form a first right bridge arm, the first left bridge arm and the first right bridge arm form an H bridge, the common connection end of the ambient temperature detection resistor RE and the heating resistor RH is grounded, the common connection end of the resistors R1 and R2 is connected to the emitter of the control tube Q1, the resistor R3 is connected to the base of the control tube Q1, and the collector of the control tube Q1 is connected to the power supply end VH; the middle position of the first left bridge arm is connected to the non-inverting input end of the operational amplifier U1, the middle position of the first right bridge arm is connected to the inverting input end of the operational amplifier U1, the capacitor C1 is connected across the inverting input end and the output end of the operational amplifier U1, the output end of the operational amplifier U1 is simultaneously connected to the resistor R3, and is connected to the base of Q1 through the other end of the resistor R3; the control tube Q1 is an NPN transistor or a MOS tube.

[0011] Furthermore, in the flow sensor driving unit, by controlling the conduction degree of Q1, the input voltage of the H-bridge is adjusted, thereby adjusting the voltage across the heating resistor RH, thereby achieving heating power control of the heating resistor RH.

[0012] Furthermore, the detection unit includes resistors R4, R5, an adjustable resistor R6, an instrumentation amplifier U2, a low-noise operational amplifier U3, reverse amplification resistors R7, R8, a feedback capacitor C2, and an ADC chip U4; the resistor R4 is connected in series with the upper thermistor RU to form a second left bridge arm, the resistor R5 and the adjustable resistor R6 are connected in series and then connected in series with the lower thermistor RD to form a second right bridge arm, and the common end of the resistors R4 and R5 is connected to the power supply VH; the other ends of the upper thermistor RU and the lower thermistor RD are grounded, and the connection between the upper thermistor RU and the resistor R4 is also connected to the instrumentation amplifier The non-inverting input terminal of the instrument amplifier U2 is connected, the connection point of the lower thermistor RD and the adjustable resistor R6 is also connected to the inverting input terminal of the instrument amplifier U2, the output terminal of the instrument amplifier U2 is connected to one end of the reverse amplifying resistor R7, the other end of the reverse amplifying resistor R7 is connected to the inverting input terminal of the low-noise operational amplifier U3, the non-inverting input terminal of the low-noise operational amplifier U3 is grounded, the capacitor C2 and the reverse amplifying resistor R8 are connected in parallel to the inverting input terminal and output terminal of the low-noise operational amplifier U3 respectively; the output terminal of the low-noise operational amplifier U3 is connected to the input terminal of the ADC chip U4.

[0013] Furthermore, in the detection unit, the resistance value of the resistor R4 is set to be equal to the sum of the resistance values of the resistor R5 and the adjustable resistor R6 connected in series; when the liquid flow rate is 0, the temperatures across the upper thermistor RU and the lower thermistor RD are the same, and the voltage difference across the upper thermistor RU and the lower thermistor RD is 0; when the liquid flow rate is not 0 and flows in the direction from the upper thermistor RU to the thermistor RD, the temperature of the lower thermistor RD is higher than the temperature of the upper thermistor RU, and when the heating resistor RH has a positive temperature coefficient, the lower thermistor RD is The resistance of the thermistor RD increases with the increase of temperature, which in turn increases the voltage across the lower thermistor RD. The voltage difference between the upper thermistor RU and the lower thermistor RD is not 0, which is amplified and detected by the instrument amplifier U2; when the heating resistor RH has a negative temperature coefficient, the resistance of the lower thermistor RD decreases with the increase of temperature, which in turn reduces the voltage across the lower thermistor RD. The voltage difference between the upper thermistor RU and the lower thermistor RD is not 0, which is amplified and detected by the instrument amplifier U2.

[0014] Furthermore, the power supply module adopts dual power supply to provide low-noise analog power supply for the flow detection circuit.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] The liquid flow path is completely isolated from the detection circuit, achieving completely contactless liquid flow detection; bidirectional flow detection function can be realized; the detection circuit is simple, the module size is small, the cost is low, and the detection accuracy is high; according to the design parameter differences of the flow sensor, a wide range of detection can be achieved; the circuit design realizes constant temperature difference control of the ambient temperature, and the influence of ambient temperature changes on the measurement is small, and no additional temperature compensation circuit is required. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic block diagram of the structure of a flow detection circuit in an embodiment of the present invention;

[0018] Figure 2 A schematic structural diagram of a flow sensor according to an embodiment of the present invention;

[0019] Figure 3 1 is a circuit diagram of a flow sensor drive unit according to an embodiment of the present invention;

[0020] Figure 4 4 is a circuit diagram of a flow detection unit in an embodiment of the present invention. DETAILED DESCRIPTION

[0021] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process. However, the protection scope of the present invention is not limited to the following embodiment.

[0022] like Figure 1 As shown, this embodiment provides a flow detection circuit based on a nanoliter flow sensor. The semiconductor flow sensor technology is used to drive and detect the flow sensor at a constant temperature to achieve the nanoliter flow detection function. The flow detection circuit includes four functional modules: a power supply module, a flow sensor drive unit, a detection unit, and a flow sensor.

[0023] A flow sensor detects flow signals. The liquid pipeline contacts the flow sensor through a well-conducting material. Three resistors are arranged in sequence on the flow sensor: RU, RH, and RD. RU is the upper thermistor, RH is the heating resistor, and RD is the lower thermistor. Heating the RH creates a constant temperature difference from the ambient temperature. Liquid flows from RU to RD. When the liquid is stationary, the thermal field in the RH is uniform, and the temperatures at the two resistors RU and RD, equidistant from the RH, are the same. When the liquid flows, the thermal field in the RH deflects toward the RD, causing the temperature at the resistor RD to be higher than that at the resistor RU. By detecting the changes in the resistance of the thermistors RD and RU, the liquid flow rate can be inferred.

[0024] In order to enable RH to produce a temperature with a constant temperature difference with the environment, an ambient temperature detection resistor RE away from the heat field is integrated on the flow sensor. This resistor is used to detect the ambient temperature. The bridge resistance of the H bridge is adjusted in the flow sensor drive unit to set the temperature difference between the two resistor positions of RE and RH. Constant temperature difference control of the RH resistor position is achieved through the closed-loop control circuit of the flow sensor drive unit.

[0025] The flow sensor drive unit drives RH heating, which is achieved through a bridge circuit. This unit mainly includes a control tube Q1, resistors R1, R2, and R3, an operational amplifier U1, a capacitor C1, and two internal resistors RE and RH of the flow sensor. R1 and RE are connected in series to form the left bridge arm, and R2 and RH are connected in series to form the right bridge arm. The common connection end of RE and RH is grounded, the common connection end of R1 and R2 is connected to the emitter of Q1, the resistor R3 is connected to the base of Q1, and the collector of Q1 is connected to the power supply VH. The middle position of the left bridge arm of the bridge is connected to the non-inverting input of U1, and the middle position of the right bridge arm is connected to the inverting input of U1. C1 is connected across the inverting input and output of U1. The output of U1 is also connected to the resistor R3, and the other end of R3 is connected to the base of Q1.

[0026] By controlling the conduction level of Q1 and adjusting the input voltage of the full bridge, the voltage across the heating resistor RH is adjusted, thereby achieving RH power control. When there is no temperature difference between the internal resistors RE and RH of the flow sensor, the difference in the resistance values of the upper bridge arm resistors R1 and R2 causes an error in the voltage across the resistors RE and RH. This error is input to the error amplifier formed by U1, and the conduction level of Q1 is adjusted through the output voltage of U1 to control the heating power of RH. Because RH is also a thermistor, its resistance changes linearly with temperature increases, ultimately making the voltages across RH and RE equal, at which point the sensor drive unit enters a steady state.

[0027] When the RH of the flow sensor is a resistor with a positive temperature coefficient, the voltage across RH is required to be lower than the voltage across RE when there is no temperature difference. Since RH is connected to the inverting input of U1, the output voltage of U1 is relatively high, which increases the conduction degree of Q1, that is, reduces the voltage difference across Q1, increases the heating power of RH, and at the same time, the resistance of RH increases due to the increase in temperature, which increases the voltage across RH, thereby forming a negative feedback regulation.

[0028] Because the flow sensor detects nanoliter flow, its size needs to be several millimeters. Therefore, the power of the heating resistor RH is relatively low, generally within 100mW, and the power requirement for Q1 is relatively low. Different usage scenarios and the difference in the size of the resistors within the flow sensor will affect the values of resistors R1 and R2. The resistors are generally set between tens of ohms and hundreds of ohms. Capacitor C1 is used to stabilize the feedback loop, and R3 is used to limit the base current of Q1. The size of R3 also slows down the change in the drive current at circuit startup, reducing the impact of the current on the flow sensor when the circuit is turned on. Q1 is an NPN transistor and is the power controller of the heating circuit. This device can also be replaced by a MOS tube to achieve the same function.

[0029] The detection unit mainly detects the resistance difference between the two bridge arm thermistors (RU and RD) of the flow sensor, converts the resistance difference into a voltage difference, and then performs gain amplification and detection.

[0030] The detection unit primarily includes precision resistors R4 and R5, an adjustable resistor R6, an instrumentation amplifier U2, a precision low-noise operational amplifier U3, U3's reverse amplifier resistors R7 and R8, U3's feedback capacitor C2, an ADC chip U4, and the flow sensor's internal thermistors RU and RD. The detection circuit is also implemented using a resistor bridge. R4 is connected in series with the flow sensor's internal resistor RU to form the left bridge arm, while R5 and R6 are connected in series with the flow sensor's internal resistor RD to form the right bridge arm. The common end of R4 and R5 is connected to power supply VH. The other ends of the flow sensor's internal resistors RU and RD are grounded. The connection between RU and R4 is also connected to the non-inverting input of instrumentation amplifier U2, while the connection between RD and R6 is also connected to the inverting input of instrumentation amplifier U2. The output of instrumentation amplifier U2 is connected to one end of R7, the other end of R7 is connected to the inverting input of U3, and the non-inverting input of U3 is grounded. C2 and R8 are connected in parallel to the inverting input and output of U3, respectively. The output of U3 is connected to the input of ADC chip U4.

[0031] Because the two resistors RU and RD are fully symmetrical, the resistance of the upper bridge arm R4 is set to be equal to the total resistance of R5 and R6 in series. When the liquid flow is 0, the temperature at both ends of RU and RD is the same, then the voltage difference between RU and RD is 0. When the liquid flow is not 0 and according to Figure 2 When the current flows in the direction of flow, the temperature of the lower bridge arm resistor RD is higher than the temperature of the resistor RU. The thermistors RU and RD of the flow sensor generally have positive temperature coefficients. Then the resistance of RD increases with the increase of temperature, and the voltage across the resistor RD increases. Then the voltage difference between RU and RD is not 0, and is amplified by the instrument amplifier U2 for detection.

[0032] When the liquid flow is reversed, the voltage across the resistor RU increases. Under the condition that the detection circuit is powered by dual power supplies, the reverse voltage can also be detected.

[0033] Instrumentation amplifier U2 detects the voltage difference between resistors RU and RD. Under normal circumstances, by configuring the resistance values of upper-arm resistors R4 and R5, the voltage difference ΔVin between RU and RD reaches a few millivolts at maximum flow. Resistor R6 is used to fine-tune the zero point, compensating for zero-point errors caused by the asymmetry of the flow sensor's internal resistors RU and RD and the asymmetry of the lead wires. The resistance of resistor R6 should not be too large, and should be set within tens of ohms to improve zero adjustment accuracy.

[0034] The instrumentation amplifier U2 is set to a magnification of G. The specific value is related to the reference source of the ADC. Since the input voltage is only at the millivolt level, the magnification G is generally set to several hundred times. The output voltage of the instrumentation amplifier U2 is Because the gain G of the instrumentation amplifier U2 is too large, in order to effectively suppress the broadband noise of the instrumentation amplifier, and at the same time drive the signal output by the instrumentation amplifier U2 with low resistance to provide it to the ADC for sampling, an inverting amplifier circuit composed of U3 is added. R7 and R8 are both precision resistors, and C2 is connected in parallel with R8. While stabilizing the feedback loop, it also forms a low-pass filter with R8 to suppress the high-frequency broadband noise of the instrumentation amplifier U2. The amplification factor of U3 is G2 = -R8 / R7. Generally, the resistance values of R8 and R7 are set to be equal, then G2 = -1, and the output voltage of U3 is In order to provide sufficient driving capability for the ADC, U3 needs to use a high-speed, high-driving capability, low-noise operational amplifier.

[0035] U4 is an ADC chip. The flow sensor can achieve a wide sampling range from 1nL to 100uL. For higher resolution, a 24-bit ADC can be used. To sample both forward and reverse flow signals through the flow sensor, a dual power supply is required, and the ADC input signal can be used to sample positive and negative voltages. If the system only needs to sample unidirectional flow, and the ADC can only sample positive voltage signals, the input signal must be raised to offset zero to sample drift.

[0036] Flow sensors need to be calibrated at zero point and full scale. Under normal circumstances, the signal output by the flow sensor is linear. As the flow rate increases, the Vout voltage increases linearly. Multi-point calibration can be used to make each flow point within the linear range of the flow sensor more accurate.

[0037] The power supply module provides low-noise power for the entire system. The VH power supply voltage should not be too high and is generally set within 10V. The analog circuit uses dual power supply to generate low-noise positive and negative power supplies.

[0038] This embodiment also provides a flow detection method, which uses the above-mentioned flow detection circuit to implement nanoliter flow detection, including the following steps:

[0039] Step 1: Use the above flow detection circuit to realize the drive and sampling of the flow sensor. The detection circuit also needs to adjust and optimize the parameters in specific applications. First, it is necessary to ensure the stability of the flow sensor drive unit and detection unit to prevent the circuit from operating in a nonlinear state. Secondly, it is necessary to adjust the resistance value of the adjustable resistor R6 so that the output voltage value of U3 is 0 when the flow rate of the flow sensor is 0, thereby making the zero point of the flow sensor accurate. Then, it is necessary to adjust the gain parameter (amplification factor G) of U2 so that the linear range of the ADC is maximized at the maximum flow rate. According to test needs, the power change of the heating resistor RH can be adjusted by adjusting the resistance values of R1 and R2 to change the sensitivity of the flow sensor.

[0040] Step 2: After the circuit parameters are adjusted to meet the requirements, the flow sensor of the present invention needs to be calibrated. Flow sensor calibration generally involves connecting the calibrated flow sensor and a standard flow sensor in series on the same flow path. Flow sensors with good linearity only need to calibrate the zero point and span. Flow sensors with poor linearity require recording multiple sets of calibration points (including the zero point and span) over the entire range, and then implementing parameter calibration over the entire range using a nonlinear fitting algorithm or a multi-segment linear algorithm.

[0041] Taking the linear algorithm calibration as an example, at the zero point (when the liquid flow rate is 0), record the flow rate of the standard flow sensor And the ADC (U4) sampling value of the calibrated flow sensor ; At the full scale position (maximum liquid flow), record the flow rate of the standard sensor And the ADC (U4) sampling value of the calibrated flow sensor ; Calculate the calibration parameters using the following formula and :

[0042] ;

[0043] ;

[0044] Step 3: When measuring, the flow sensor of the present invention needs to use the sampling value of ADC (U4) and calibration parameters 、 Calculate real-time traffic values , real-time traffic value Calculated by the following formula:

[0045] ;

[0046] The above formula only applies to a two-point calibration example for a sensor with unidirectional flow and good linearity. For bidirectional flow, a reverse calibration parameter needs to be added. If linearity is poor, the ideal calibration algorithm needs to be selected based on actual calibration data, as there are many different calibration algorithms.

[0047] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A flow detection circuit based on a nanoliter flow sensor, characterized in that: include: Flow sensor, flow sensor drive unit, detection unit and power supply module; The flow sensor is respectively connected to the flow sensor drive unit and the detection unit, and the power supply module is respectively connected to the flow sensor drive unit and the detection unit, and is used to power the flow sensor drive unit and the detection unit; the flow sensor is a nanoliter flow sensor, which is in contact with the liquid pipeline through a thermally conductive material and is used to detect and obtain a flow signal. The flow sensor includes an upper thermistor RU, a heating resistor RH, a lower thermistor RD and an ambient temperature detection resistor RE; the heating resistor RH and the ambient temperature detection resistor RE are connected to the flow sensor drive unit, and the flow sensor drive unit is used to drive the heating resistor RH to achieve heating; the upper thermistor RU and the lower thermistor RD are connected to the detection unit, and the detection unit is used to detect the resistance difference between the upper thermistor RU and the lower thermistor RD and convert it into a voltage difference, and then amplify and detect it.

2. A flow detection circuit based on a nanoliter flow sensor according to claim 1, characterized in that: In the flow sensor, heating by the heating resistor RH generates a temperature with a constant temperature difference from the environment, and the direction of liquid flow is from the upper thermistor RU to the lower thermistor RD. When the liquid is stationary, the thermal field of the heating resistor RH is uniform, and the temperatures of the upper thermistor RU and the lower thermistor RD, which are equidistant from the heating resistor RH, are the same. When the liquid flows, the thermal field of the heating resistor RH deflects toward the lower thermistor RD, and the temperature of the lower thermistor RD is higher than that of the upper thermistor RU. By detecting the resistance changes of the lower thermistor RD and the upper thermistor RU, the liquid flow signal can be calculated.

3. A flow detection circuit based on a nanoliter flow sensor according to claim 2, characterized in that: In the flow sensor, the upper thermistor RU and the lower thermistor RD have equal resistance values and are positioned symmetrically with respect to the heating resistor RH.

4. A flow detection circuit based on a nanoliter flow sensor according to claim 3, characterized in that: In the flow sensor, the ambient temperature detection resistor RE is used to detect the ambient temperature. The resistance value of the heating resistor RH in the H-bridge is adjusted in the flow sensor drive unit to achieve the setting of the temperature difference between the two resistor positions of the ambient temperature detection resistor RE and the heating resistor RH. The constant temperature difference control of the heating resistor RH position is achieved through the closed-loop control of the flow sensor drive unit.

5. A flow detection circuit based on a nanoliter flow sensor according to claim 4, characterized in that: The flow sensor drive unit includes a control tube Q1, resistors R1, R2, and R3, an operational amplifier U1, and a capacitor C1; wherein, the resistor R1 and the ambient temperature detection resistor RE are connected in series to form a first left bridge arm, the resistor R2 and the heating resistor RH are connected in series to form a first right bridge arm, the first left bridge arm and the first right bridge arm form an H bridge, the common connection end of the ambient temperature detection resistor RE and the heating resistor RH is grounded, the common connection end of the resistors R1 and R2 is connected to the emitter of the control tube Q1, the resistor R3 is connected to the base of the control tube Q1, and the collector of the control tube Q1 is connected to the power supply terminal VH; the middle position of the first left bridge arm is connected to the non-inverting input terminal of the operational amplifier U1, the middle position of the first right bridge arm is connected to the inverting input terminal of the operational amplifier U1, the capacitor C1 is connected across the inverting input terminal and the output terminal of the operational amplifier U1, the output terminal of the operational amplifier U1 is simultaneously connected to the resistor R3, and is connected to the base of Q1 through the other end of the resistor R3; the control tube Q1 is an NPN transistor or a MOS transistor.

6. A flow detection circuit based on a nanoliter flow sensor according to claim 5, characterized in that: In the flow sensor drive unit, the conduction degree of Q1 is controlled to adjust the input voltage of the H bridge, thereby adjusting the voltage across the heating resistor RH to achieve heating power control of the heating resistor RH.

7. The flow detection circuit based on a nanoliter flow sensor according to claim 5, characterized in that: The detection unit includes resistors R4 and R5, an adjustable resistor R6, an instrumentation amplifier U2, a low-noise operational amplifier U3, reverse amplification resistors R7 and R8, a feedback capacitor C2, and an ADC chip U4; the resistor R4 is connected in series with the upper thermistor RU to form a second left bridge arm, the resistor R5 and the adjustable resistor R6 are connected in series and then connected in series with the lower thermistor RD to form a second right bridge arm, and the common end of the resistors R4 and R5 is connected to the power supply VH; the other ends of the upper thermistor RU and the lower thermistor RD are grounded, and the connection between the upper thermistor RU and the resistor R4 is also connected to the instrumentation amplifier U2 The positive input terminal of the low-noise operational amplifier U3 is connected to the positive input terminal of the low-noise operational amplifier U3, the connection point between the lower thermistor RD and the adjustable resistor R6 is also connected to the inverting input terminal of the instrumentation amplifier U2, the output terminal of the instrumentation amplifier U2 is connected to one end of the reverse amplification resistor R7, the other end of the reverse amplification resistor R7 is connected to the inverting input terminal of the low-noise operational amplifier U3, the positive input terminal of the low-noise operational amplifier U3 is grounded, the capacitor C2 and the reverse amplification resistor R8 are connected in parallel to the inverting input terminal and output terminal of the low-noise operational amplifier U3 respectively; the output terminal of the low-noise operational amplifier U3 is connected to the input terminal of the ADC chip U4.

8. The flow detection circuit based on the nanoliter flow sensor according to claim 7, characterized in that: In the detection unit, the resistance value of the resistor R4 is set to be equal to the sum of the resistance values of the resistor R5 and the adjustable resistor R6 connected in series; when the liquid flow rate is 0, the temperatures at both ends of the upper thermistor RU and the lower thermistor RD are the same, and the voltage difference between the upper thermistor RU and the lower thermistor RD is 0; when the liquid flow rate is not 0 and flows in the direction from the upper thermistor RU to the lower thermistor RD, the temperature of the lower thermistor RD is higher than the temperature of the upper thermistor RU, and when the heating resistor RH has a positive temperature coefficient, the lower thermistor The resistance of RD increases with the increase of temperature, which in turn causes the voltage across the lower thermistor RD to increase. The voltage difference between the upper thermistor RU and the lower thermistor RD is not 0, and is amplified and detected by the instrument amplifier U2; when the heating resistor RH has a negative temperature coefficient, the resistance of the lower thermistor RD decreases with the increase of temperature, which in turn causes the voltage across the lower thermistor RD to decrease. The voltage difference between the upper thermistor RU and the lower thermistor RD is not 0, and is amplified and detected by the instrument amplifier U2.

9. The flow detection circuit based on the nanoliter flow sensor according to claim 8, characterized in that: The power supply module adopts dual power supply to provide low-noise analog power supply for the flow detection circuit.

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