Flow sensing structure and temperature drift compensation circuit for flow sensing structure

Abstract

The invention discloses a flow sensing structure and a temperature excursion compensation circuit for the flow sensing structure, the temperature excursion compensation circuit comprises a first resistor, a second resistor, an intelligent temperature control resistor, an environment temperature reference resistor and a micro heater which are sequentially connected in series, the connection point of the first resistor and the intelligent temperature control resistor is connected with the positive input end of a first operational amplifier, and the negative input end of the second operational amplifier is connected with the negative input end of a second operational amplifier; the connection point of the second resistor and the micro heater is connected with the negative input end of the first operational amplifier, and the connection point of the first resistor and the second resistor is connected with the output end of the first operational amplifier; the micro heater is in heat transfer with the Wheatstone bridge in a thermal coupling mode, the first end and the second end of the Wheatstone bridge are connected with the negative input end and the positive input end of the second operational amplifier respectively, the positive output end of the second operational amplifier is connected with the first end of the first configuration resistor, and the negative output end of the second operational amplifier is connected with the second end of the second configuration resistor. The temperature drift can be remarkably suppressed, and the measurement precision is improved.

Description

Technical Field

[0001] The present invention relates to the field of sensor technology, and in particular to a flow sensing structure and a temperature drift compensation circuit for the flow sensing structure. Background Art

[0002] Flow measurement is crucial in the industrial, medical, and environmental sectors. Industrial production feed and waste gas monitoring, the operation of medical equipment (such as ventilators, anesthesia machines, and microfluidic injection devices), and environmental HVAC (Heating, Ventilation, and Air Conditioning) system regulation all rely on accurate flow data. In recent years, the rapid development of smart buildings, precision medicine, and the semiconductor industry has created an urgent need for high-precision microflow monitoring technology. The development of high-performance CMOS-MEMS integrated thermal flow sensor SoCs (System on a Chip) faces challenges: its detection involves multi-domain signal conversion, requiring the coordinated design of MEMS (Micro-Electro-Mechanical Systems) and ASICs (Application-Specific Integrated Circuits); in common application scenarios, thermal flow sensors are susceptible to ambient temperature influences, resulting in signal drift, requiring temperature drift compensation of the integrated SoC to reduce calibration work and improve temperature-dependent measurement accuracy. Therefore, conducting research on the coordinated design of integrated flow sensor SoC and temperature drift suppression under multi-field coupling is of great significance to meeting the needs of micro-flow monitoring and promoting the development of high-precision integrated flow sensor technology.

[0003] Over the past two decades, significant progress has been made in temperature drift compensation technology for thermal flow sensor systems. The relevant technologies can be mainly divided into two categories: hardware circuit compensation and software algorithm compensation. Hardware compensation solutions typically employ additional temperature sensors coupled with compensation circuits, while software compensation encompasses strategies such as semi-empirical model construction and programmable resistor adjustment. However, all of the above methods have limitations: on the one hand, existing compensation solutions generally ignore the sensitivity of system-level components such as signal conditioning circuits to temperature changes; on the other hand, due to the inadequacy of electronic design automation (EDA) full-system simulation technology, accurate modeling and prediction of temperature coupling effects are difficult to achieve. Therefore, current thermal flow sensors still generally face technical bottlenecks such as significant temperature drift and insufficient measurement accuracy. Summary of the Invention

[0004] The embodiments of the present invention provide a flow sensor structure and a temperature drift compensation circuit for the flow sensor structure, aiming to solve the problems of significant temperature drift and insufficient measurement accuracy that are still commonly faced by current thermal flow sensors.

[0005] In a first aspect, an embodiment of the present invention discloses a temperature drift compensation circuit for a flow sensor structure, wherein the temperature drift compensation circuit includes a control circuit and a readout circuit; The control circuit includes a first resistor, a second resistor, an intelligent temperature-controlled resistor, an ambient temperature reference resistor, a microheater, and a first operational amplifier; wherein the first resistor, the second resistor, the intelligent temperature-controlled resistor, the ambient temperature reference resistor, and the microheater are sequentially arranged in series, the connection point between the first resistor and the intelligent temperature-controlled resistor is connected to the positive input terminal of the first operational amplifier, the connection point between the second resistor and the microheater is connected to the negative input terminal of the first operational amplifier, and the connection point between the first resistor and the second resistor is connected to the output terminal of the first operational amplifier; The readout circuit includes a Wheatstone bridge, a second operational amplifier, a first configuration resistor, a gain control resistor and a second configuration resistor; wherein the microheater transfers heat with the Wheatstone bridge through thermal coupling, the first end of the Wheatstone bridge is connected to the negative input end of the second operational amplifier, the second end of the Wheatstone bridge is connected to the positive input end of the second operational amplifier, the positive output end of the second operational amplifier is connected to the first end of the first configuration resistor, the negative output end of the second operational amplifier is connected to the second end of the second configuration resistor, and the gain control resistor is arranged in series between the first configuration resistor and the second configuration resistor.

[0006] The Wheatstone bridge comprises four bridge arms connected end to end, and each bridge arm is provided with a temperature sensing element.

[0007] The intelligent temperature-controlled resistor is a resistor with a positive temperature coefficient.

[0008] The readout circuit includes a first fixed-value resistor and a second fixed-value resistor. The first fixed-value resistor is arranged in parallel with the gain control resistor, and the second fixed-value resistor is arranged in series between the gain control resistor and the second configuration resistor.

[0009] The gain control resistor is a resistor with a negative temperature coefficient.

[0010] The micro heater has a serpentine structure.

[0011] In a second aspect, an embodiment of the present invention further discloses a flow sensing structure, which includes a substrate and the temperature drift compensation circuit, wherein the temperature drift compensation circuit is arranged on the substrate.

[0012] A cavity is provided on the substrate, the four temperature sensing elements and the micro heater are all provided on the cavity, and the four temperature sensing elements are symmetrically provided on both sides of the micro heater.

[0013] The micro heater is arranged on a support member, both ends of the support member are connected to support beams respectively, and a wire electrically connected to the micro heater is arranged on the upper end surface of one of the support beams.

[0014] The substrate is a silicon substrate.

[0015] The present invention discloses a flow sensing structure and a temperature drift compensation circuit for the flow sensing structure, wherein the temperature drift compensation circuit includes a control circuit and a readout circuit; the control circuit includes a first resistor, a second resistor, an intelligent temperature-controlled resistor, an ambient temperature reference resistor, a microheater and a first operational amplifier; wherein the first resistor, the second resistor, the intelligent temperature-controlled resistor, the ambient temperature reference resistor and the microheater are sequentially arranged in series, the connection point between the first resistor and the intelligent temperature-controlled resistor is connected to the positive input terminal of the first operational amplifier, the connection point between the second resistor and the microheater is connected to the negative input terminal of the first operational amplifier, and the connection point between the first resistor and the second resistor is connected to the negative input terminal of the first operational amplifier. The microheater is connected to the output end of the first operational amplifier; the readout circuit includes a Wheatstone bridge, a second operational amplifier, a first configuration resistor, a gain control resistor, and a second configuration resistor; wherein the microheater transfers heat to the Wheatstone bridge via thermal coupling, the first end of the Wheatstone bridge is connected to the negative input end of the second operational amplifier, the second end of the Wheatstone bridge is connected to the positive input end of the second operational amplifier, the positive output end of the second operational amplifier is connected to the first end of the first configuration resistor, the negative output end of the second operational amplifier is connected to the second end of the second configuration resistor, and the gain control resistor is arranged in series between the first configuration resistor and the second configuration resistor. The embodiment of the present invention adopts intelligent temperature-controlled resistors for configuration, so that the temperature difference output of the control circuit is a variable temperature difference. This design can significantly suppress temperature drift, solve the influence of temperature drift on measurement results, and improve measurement accuracy and reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 A circuit structure diagram of a temperature drift compensation circuit provided in an embodiment of the present invention; Figure 2 A virtual model diagram of a temperature drift compensation circuit provided by an embodiment of the present invention; Figure 3 A structural diagram of a flow sensor structure provided by an embodiment of the present invention; Figure 4 A heat transfer analysis diagram of a microheater provided in an embodiment of the present invention; Figure 5 A top view of a micro heater provided in an embodiment of the present invention; Figure 6 A side view of a micro heater provided in an embodiment of the present invention; Figure 7 A schematic diagram of the effect of the flow sensor structure provided by an embodiment of the present invention in VTD mode; Figure 8 A circuit structure diagram of a control circuit provided in another embodiment of the present invention; Figure 9 A schematic diagram of the flow sensor structure provided by an embodiment of the present invention; Figure 10 A schematic diagram showing the effects of the flow sensor structure provided by an embodiment of the present invention at different ambient temperatures; Figure 11 A schematic diagram of an experimental configuration of a flow sensor structure provided in an embodiment of the present invention; Figure 12 A schematic diagram of the effect of the flow sensor structure provided by an embodiment of the present invention under a traditional configuration; Figure 13 A schematic diagram of the effect of the flow sensor structure provided by an embodiment of the present invention under a variable temperature difference drive configuration; Figure 14 A schematic diagram of the effect of the flow sensor structure provided by an embodiment of the present invention under the intelligent gain control configuration.

[0018] Reference numerals: 10, flow sensing structure; 11, control circuit; 12, readout circuit; 13, substrate; R1, first resistor; R2, second resistor; R c , intelligent temperature control resistor; R r , ambient temperature reference resistance; R h , micro heater; OPA, first operational amplifier; CFIA, second operational amplifier; R n , first configuration resistor; R g , gain control resistor; R p , the second configuration resistor; L b , support beam; R gp , the first fixed value resistor; R gs , a second fixed value resistor. DETAILED DESCRIPTION

[0019] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0020] It will be understood that when used in this specification and the appended claims, the terms “comprises” and “comprising” indicate the presence of described features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0021] It should also be understood that the terms used in the present specification are only for the purpose of describing particular embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, the singular forms "a", "an", and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0022] It should be further understood that the term "and / or" used in the present description and appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.

[0023] The embodiment of the present invention discloses a temperature drift compensation circuit for a flow sensor structure 10, such as Figure 1 As shown, the temperature drift compensation circuit includes a control circuit 11 and a readout circuit 12; the control circuit 11 includes a first resistor R1, a second resistor R2, and an intelligent temperature control resistor R c , ambient temperature reference resistance R r 、Micro heater R h and a first operational amplifier OPA; wherein the first resistor R1, the second resistor R2, the intelligent temperature control resistor R c , the ambient temperature reference resistor R r , the micro heater R h The first resistor R1 and the intelligent temperature control resistor R c The connection point between them is connected to the positive input terminal of the first operational amplifier OPA, and the second resistor R2 is connected to the micro heater R h The connection point between the first resistor R1 and the second resistor R2 is connected to the negative input terminal of the first operational amplifier OPA, and the connection point between the first resistor R1 and the second resistor R2 is connected to the output terminal of the first operational amplifier OPA; the readout circuit 12 includes a Wheatstone bridge, a second operational amplifier CFIA, a first configuration resistor R n , gain control resistor Rg and the second configuration resistor R p ; Wherein, the micro heater R h Heat is transferred to the Wheatstone bridge by thermal coupling, the first end of the Wheatstone bridge is connected to the negative input end of the second operational amplifier CFIA, the second end of the Wheatstone bridge is connected to the positive input end of the second operational amplifier CFIA, and the positive output end of the second operational amplifier CFIA is connected to the first configuration resistor R n The first end of the second operational amplifier CFIA is connected to the negative output end of the second configuration resistor R p The second end of the gain control resistor R g The first configuration resistor R is set in series n And the second configuration resistor R p between.

[0024] In this embodiment, R1 and R2 satisfy formula (1): (1); In formula (1), k is the ratio of the resistance values of the two branches when the Wheatstone bridge is balanced, R 1,2 Configure the resistor for the bridge ratio, R r is the ambient temperature reference resistance, the micro heater R h Also the resistance of the micro heater, R c,const Configure the resistor for overheating temperature. Further, the ambient temperature reference resistor R r and the resistance of the micro heater R h Satisfy the following formula: (2); Where R r0 and R h0 Respectively represent the resistance values of the ambient temperature reference resistor and the micro heater at an ambient temperature of T0 = 25 °C, T a Represents the ambient temperature, T h represents the heating temperature of the micro heater, α r and α h They represent the TCR of the ambient temperature reference resistor and the micro heater at an ambient temperature of T0 = 25 °C. Since both are resistors implemented by the p+ polysilicon layer, ignoring the process error, it can be considered that α r = α h , take the average when calculating, use express.

[0025] In order to make the micro heater R h The operating temperature is a fixed temperature difference Δ relative to the ambient temperature T h= T h - T a . Based on this, configure R in circuit design r0 / R h0 = k , combined with formula (1) and (2), it can be calculated that the overheat temperature configuration resistor R c,const The resistance value should be: (3); For this design, the measured values are R1 = 2560 Ω, R2 = 512 Ω, and R h0 = 1018.9 Ω, α h = 2.944 × 10 −3 ℃ −1 , R r0 = 5104.3 Ω, α r = 2.830 × 10 −3 ℃ −1 , Δ T h = 50℃.

[0026] More specifically, the second operational amplifier is an instrumentation operational amplifier, which is used to amplify small voltages in the μV to mV range.

[0027] Specifically, the Wheatstone bridge includes four bridge arms connected end to end, and each bridge arm is provided with a temperature sensing element.

[0028] In a more specific embodiment, for the readout circuit 12, four upstream and downstream temperature sensing elements are configured in a Wheatstone bridge, and the bridge supply voltage is V s , the voltage difference Δ at both ends of the bridge is very small in mV V After amplification by CFIA, the output differential voltage V out To characterize the flow rate. The design of the four temperature sensing elements is exactly the same. Due to the symmetry of the structure, the resistance of the upstream temperature sensing element and the resistance of the downstream temperature sensing element can be expressed as R u = R u1,2 and R d = R d1,2 , the resistance values are both R s0 For subsequent applications, the average values of the resistance and TCR of the four temperature sensing elements at 25°C are taken as follows: R s0= 4055.7 Ω and α s =2.897 × 10 −3 ℃ −1 At the same time, the CFIA chopping frequency in the readout circuit 12 is set to 20 kHz. After CFIA amplification, the output of the readout circuit 12 can be expressed as: (4); Where, under normal configuration, the supply voltage of the Wheatstone bridge is V s = 1 V, G = ( R n + R g + R p ) / R g is the closed-loop gain of CFIA, R n , R g , R p They are all gain configuration resistors. Under normal configuration, G = 200.

[0029] The present invention also discloses a flow sensor structure 10, such as Figure 2 As shown, the flow sensing structure 10 includes the temperature drift compensation circuit and a substrate 13 , and the temperature drift compensation circuit is disposed on the substrate 13 .

[0030] Specifically, the micro heater R h The base 13 is provided with a cavity, the four temperature sensing elements and the micro heater R h are all arranged on the cavity, and the four temperature sensing elements are symmetrically arranged on the micro heater R h On both sides; the micro heater R h It is arranged on the support member, and the two ends of the support member are connected to the support beam respectively, and the micro heater R h The electrically connected wires are arranged on the upper end surface of one of the support beams; and the substrate is a silicon substrate.

[0031] In a more specific embodiment, a standard CMOS-MEMS process is used to realize the monolithic integration of the flow sensing structure 10. A system-level simulation model is constructed based on the geometric parameters, material properties and electrothermal coupling characteristics of the flow sensing structure. The system-level simulation model consists of three parts: 1. Microheater R h1. Equivalent Circuit Model (ECM), i.e., microheater model; wherein, the microheater model is constructed based on the microheater; 2. Heat transfer model; wherein, the heat transfer model is constructed based on the heat transfer relationship between the microheater and the Wheatstone bridge; 3. Equivalent circuit model of the temperature sensing element, i.e., the temperature sensing element model; wherein, the temperature sensing element model is constructed based on four temperature sensing elements; more specifically, in the system-level simulation model, R c Depend on R c0 and α c Common representation; among them, R c0 is the resistance of the intelligent temperature control resistor at 25°C, α c is the temperature coefficient of resistance.

[0032] In order to construct the ECM of the thermistor, it is necessary to determine the microheater R under fluid flow conditions. h At a given operating temperature T Ambient temperature distribution under T ( x ). To this end, a heat transfer model needs to be proposed, which takes into account the key heat transfer behaviors; considering that the micro heater and four temperature sensing elements will be encapsulated in the flow channel, the heat transfer model can be expressed as: (5); Where, T ( x ) indicates along x The temperature distribution in the direction (A-A' direction) x The direction is also the direction of fluid flow; k s and k f Represent the thermal conductivity of the film and fluid respectively; the materials of the micro heater and the temperature sensing element are both thin films; t f Indicates the thickness of the film; the thickness of the micro heater is the same as the thickness of the temperature sensing element; H ca and H ch represent the height of the cavity and the encapsulation flow channel respectively; ρ f 、 c f and U represent the density, heat capacity and flow rate of the fluid respectively; δ t represents the average thermal boundary layer thickness. T (x ), δ t 、 ρ f 、 c f and k f The values are related to the ambient temperature T a Related.

[0033] Boundary conditions that limit temperature distribution T (- l ca ) = T ( l ca ) = 0 and T (- w h ) = T ( w h ) = Δ T h , we can get the following analytical solution for the temperature distribution: (6); Where, 2 w h is the width of the microheater, 2 l ca It is hollow x Direction length, is the eigenvalue of formula (5), is the coefficient vector of the equation on the left side of equation (5). From this, the upstream and downstream temperature distribution can be calculated as: (7); Where, D hs is the distance between the micro heater and the center of the temperature sensing unit.

[0034] ECM of temperature sensing element: (8); Where, R s Indicates the upstream thermistor R u1,2 or downstream thermistor R d1,2 The resistance value, R s0 Room temperature ( TThe ECM of the temperature sensing element is implemented in the EDA (Electronic Design Automation) platform in Verilog-A format. This allows the creation of an ECM that describes the temperature sensing element based on the heat transfer model.

[0035] To achieve fully coupled simulation of the microheater and temperature sensor within the EDA platform, it is necessary to construct an ECM for the microheater to accurately predict its heating temperature under ASIC (Application-Specific Integrated Circuit) stimulation. Based on the principle of energy conservation and the semi-empirical King's Law, the transient power consumption of the microheater is: (9); Where, ρ h 、 ν h and c h represent the density, volume and heat capacity of the microheater respectively, t For time. In addition, A indicates that the heat loss due to conduction is taken into account, and B represents the forced convection caused by the boundary layer flow. Both factors are temperature-dependent. In order to calculate these two parameters, it is necessary to analyze the heat exchange between the microheater and the outside world.

[0036] Consider the heat exchange between the micro heater and the surrounding environment, such as Figures 4 to 6 As shown, the steady-state energy conservation of the microheater can be described as: (10); Where, the superscripts “conv” and “cond” represent heat convection and heat conduction, respectively; the subscripts “s” and “f” represent film and fluid, respectively; and “s1” and “s2” represent the support beam L. b With and without circuit wires. In steady state, we can get: (11); Where, L h represents the length of the micro heater; k s1 and k s2 denote the thermal conductivity of the two supporting beams respectively; W b and L b Respectively represent the width and length of the support beam; 2 l cIt is the edge of the chip x the length of the shaft; μ f Indicates the dynamic viscosity of the fluid and is also related to the ambient temperature T a Related.

[0037] like Figure 2 As shown, in order to calculate the overheating temperature of the micro heater under actual circuit conditions, the power of the micro heater P h Equivalent to a current generated by a voltage-controlled current source (VCCS) I VCCS .at the same time, Equivalent to a resistor R t ( U ),and ρ h ν h c h Equivalent to a capacitor C t , target overheat temperature Δ T h is equivalent to a resistor R t ( U ) and capacitors C t In addition, due to the resistance of the micro heater R h The voltage across the microheater can be modeled as a voltage-controlled voltage source (VCVS) by using a form similar to formula (8). V VCCS = V h = I h R h ( T h ). All these expressions can be implemented in the EDA platform using Verilog-A.

[0038] Temperature drift compensation based on control circuit: using resistors with positive temperature coefficient (PTC) R cThe configuration is made so that the temperature difference output of the control circuit 11 is a variable temperature difference (VTD). This design makes the overheat temperature Δ T h It has a positive temperature coefficient characteristic, thereby effectively compensating for the negative temperature drift problem output by the readout circuit 12. In this configuration, the variable temperature difference control result can be calculated as: (12); Where, α c 、 α h and α r Respectively represent the micro heater overheating temperature regulation and intelligent temperature control resistor R c , micro heater R h and ambient temperature reference resistor R r TCR, R c0 、 R h0 and R r0 These components are at the reference temperature T 0 = Resistance at 25°C. Select from 700 to 800 Ω R c Initial resistance R c0 In order to achieve R c Based on the typical temperature coefficient range of PTC resistors in commercial 0.18 μm1P6M CMOS process (2 × 10 -3 ~ 4 × 10 −3 ℃ -1 ), confirmed α c The value range of .

[0039] In order to evaluate the effect of ambient temperature change on the output of the readout circuit 12 in VTD mode and determine the intelligent temperature control resistor R c The range of resistance and TCR can be selected at an ambient temperature of 25°C to achieve R c On-chip integration. R cA linear programming (LP) model was established for range evaluation to determine the selection of its parameters. The temperature drift optimization objective of the configuration model was: (13); Set the maximum drift error tolerance ε var 1% to expand the possible selection range of intelligent temperature control resistors. Variation With intelligent temperature control resistor R c Initial resistance at 25°C R c0 and the temperature coefficient of resistance α c The LP model can be expressed as follows: (14); In this parameter space, the overcompensation (ie, Variation > 0) are shown as red areas, and the undercompensation (ie, Variation < 0) area is shown as blue area. To configure the compensation resistor, select the parameters in the colored area. The configuration is achieved at 25 ℃. R c0 = 748.21 Ω, α c = 3.037 × 10 -3 ℃ -1 , which can be realized by silicided n+ diffusion resistors in a commercial 0.18 μm1P6M CMOS process and monolithically integrated into the flow sensing structure 10.

[0040] like Figure 7 In the system-level simulation model, overheat temperature control and intelligent temperature control resistors are R c The configuration settings are R c0 = 748.21 Ω, α c = 3.037 × 10 -3 ℃ -1 In the Spectre simulation environment, parameter sweeps were performed at different temperatures (0°C, 25°C, and 50°C) to evaluate the overheat temperature control performance and output characteristics of the flow sensing structure 10. The flow sensing structure 10 was comprehensively evaluated within the flow velocity range of 0 to 10 m / s. The simulation results show that in VTD mode, the overheat temperature Δ T h With ambient temperature T aRaise and automatically lower ( Figure 7 (a)). At a flow rate of 10 m / s, the temperature drift of the readout circuit output is significantly reduced from ±7.9% to Figure 7 ±0.6% as shown in (b).

[0041] Temperature drift compensation based on intelligent gain control: If the gain G Can increase with temperature (ie, gain G With a positive temperature coefficient, it can also compensate for the negative temperature drift problem of the readout circuit output. G =( R n + R g + R p ) / R g To make it have positive temperature characteristics, R g Starting with the negative temperature coefficient (NTC) resistor, we can achieve intelligent gain control and configure the required intelligent gain control resistor. R g,tot ( T a ).

[0042] For resistors such as MF52, the Steinhart-Hart formula is more accurate. The specific description model of its resistance temperature characteristics is: (15); In the formula, all temperatures are in degrees Celsius (℃). R g0 represents the initial resistance value at 25°C, B The value is the material constant of the resistor.

[0043] However, only R g ( T a ) is not enough to achieve R g,tot ( T a The reason is that the slope and curvature of the temperature-varying curve of the resistor finally configured need to be flexibly adjusted according to the temperature drift characteristics of the output of different readout circuits. R g,tot ( T a ) nonlinear adjustment of temperature characteristics, the present invention adoptsR g ( T a ) first with a fixed value resistor R gp Then connect it in parallel with another fixed value resistor R gs Cascade strategy.

[0044] Specifically, such as Figure 8 As shown, the readout circuit 12 includes a first fixed resistor R gp And the second fixed resistor R gs , the first fixed value resistor R gp With the gain control resistor R g The second fixed value resistor R gs Set in series with the gain control resistor R g And the second configuration resistor R p The gain control resistor R g is a resistor with a negative temperature coefficient; Through this combination, a resistor is constructed that can achieve intelligent control of the total gain: (16); The original value R g Replace with R g,tot ( T a ) after which the readout circuit 12 is subjected to R g,tot ( T a ) The gain after adjustment is: (17); In order to make the readout circuit output voltage Variation Close to 0%, need to achieve: (18); Where, V out* ( T a ) represents the readout circuit output after regulation, V out ( T a ) is the readout circuit output before compensation, which is determined by the output result of the uncompensated system-level simulation. G ( T a = 25 ℃) is the fixed gain when not compensated G= 200. At this time, in order to achieve R g,tot ( T a ), the optimization objective can be expressed as: (19); Where, ε stop To solve the algorithm convergence value (this design is 10 -6 The optimization objective is solved using the scipy.optimize.minimize optimizer in Python. The specific algorithm used is the Limited-memory Broyden-Fletcher-Goldfarb-Shanno with Bound Constraints (L-BFGS-B). The L-BFGS-B algorithm iterates the optimization objective (19) and obtains R gs = 1656 Ω, R gp = 720 Ω, R g0 =660 Ω. In practical applications, by connecting one 1 kΩ, one 5 kΩ, and one 4 kΩ commercial MF52 resistor in parallel, a resistance of 689.66 Ω can be obtained. R g0 .in addition, R gs Take 1656 Ω, R gp Take 720 Ω. In the actual circuit, a precision potentiometer can be used to achieve the corresponding high-precision resistance value, making it easier to configure the intelligent gain control resistor.

[0045] After clarifying the theoretical model and key parameters, the solution is further verified through system-level simulation. In the system-level simulation model, according to the configuration results above, the intelligent gain control resistor is set to R g,tot The configuration settings are B =3825 K, R g0 = 689.66 Ω, R gs = 1656 Ω and R gp = 720 Ω. Using the Spectre simulation tool, we set different temperatures (0 ℃, 25 ℃ and 50 ℃) to perform parameter sweeps and simulate the system gain and output. R g,tot (T a ) and the corresponding gain G ( T a )like Figure 9 (a) and (b). Similarly, the entire sensing system was comprehensively evaluated in the flow rate range of 0 ~ 10 m / s. The simulation results show that under the configuration of the intelligent gain control circuit, the system gain shows a nonlinear downward trend with the increase of temperature, and the intelligent readout circuit output. At a flow rate of 10 m / s, the temperature drift of the readout circuit output is significantly reduced from ±7.9% to Figure 10 ±0.14% shown.

[0046] In summary, the flow sensor structure design provided by the present invention can significantly suppress temperature drift and solve the impact of temperature drift on measurement results. Specific embodiments are as follows: like Figure 11 As shown, the fabricated flow sensing structure 10 was encapsulated in a 3D-printed flow channel measuring 65 mm × 12 mm × 2.5 mm (length × width × height) and embedded in a printed circuit board (PCB). Nitrogen (N2) gas flow was released from a cylinder, reduced in pressure by a first-stage pressure reducing valve, and then delivered to a gas mass flow controller, where the flow rate was controlled by a computer. The flow then passed through a throttle valve and reached a reference flow meter. Throughout the experiment, the flow sensing structure, encapsulated in the flow channel and system PCB, was placed in a constant temperature chamber to maintain a stable ambient temperature. Furthermore, to ensure that the temperature of the N2 gas flow was consistent with the set ambient temperature in the chamber, the N2 gas passed through a long section of tubing within the constant temperature chamber to stabilize the temperature before entering the 3D-printed flow channel. The DUT was powered by a benchtop power supply, with a 20 kHz chopping clock provided by a function generator. A benchtop multimeter was used to measure the output of the reference flow meter, the output of the control circuit OPA, the voltage across the microheater, and the output of the readout circuit.

[0047] Configure the overheat temperature control resistor of the DUT control circuit to be constant R c = 750 Ω, the readout circuit gain control resistor is configured as a constant R g = 2 kΩ, the bridge bias voltage is configured to be constant V s = 1 V, which is the traditional configuration. The MFC device was controlled to vary the flow rate from 0 to 18 SLM, corresponding to a flow rate of 0 to 10 m / s at the sensor in the pipe. The thermostat was controlled to conduct experiments at 0°C, 25°C, and 50°C, and the voltage across the microheater was measured and recorded. Vh , OPA output voltage V op and output voltage V out Through formula (8), the proportional control resistor on the right side of the control circuit R 2, the voltage across the micro heater V h and OPA output voltage V op , the heating temperature of the micro heater can be calculated as: (20); From this, the superheat temperature Δ can be calculated T h = T h - T a .

[0048] In the traditional configuration, the test results of the overheat temperature of the micro heater and the output of the readout circuit at ambient temperatures of 0 ℃, 25 ℃ and 50 ℃ are as follows: Figure 12 As shown in the figure. The experimental results show that, in the control circuit, the CTD circuit can effectively drive the microheater as the flow rate and temperature change, with an overheat temperature error of 1.2°C. Finally, in the readout circuit output, the readout circuit output has an error of ±8.9% at a flow rate of 9 m / s under ambient temperature variations of 0 to 50°C. Experimental test results show that due to the limited output swing of the CFIA, the output at an ambient temperature of 0°C exhibits premature saturation at high flow rates. Furthermore, due to variations in micromachining processes, the resistance values and TCRs of the four temperature-sensing elements at 25°C vary. These manufacturing variations make it difficult to account for the specific resistance parameters of each chip in simulations, and only consistent resistance values and TCRs can be used for simulation, resulting in a discrepancy of approximately 1% between the simulation results and the experimental results.

[0049] In the temperature drift compensation strategy based on variable temperature difference drive configuration, in order to configure the control circuit in VTD mode, it is necessary to set the fixed value of the overheat temperature control resistor R c Replace with a resistor with temperature drift compensation function, the resistance at 25 ℃ is R c0 =748.21 Ω, TCR is α c = 3.037 × 10 -3 ℃ -1 . After configuring the VTD control mode, experimental tests were conducted at ambient temperatures ranging from 0 to 50°C and flow rates from 0 to 10 m / s. The experimental results are as follows: Figure 13 After applying the VTD mode, the overheating temperature of the micro heater Δ T h As the ambient temperature rises, the output temperature increases, indicating that the control circuit correctly configures the microheater's overheat temperature. Ultimately, the overheat temperature compensates for the negative temperature drift of the output due to the positive temperature coefficient, reducing it to ±1.6% at 10 m / s.

[0050] In the temperature drift compensation strategy based on intelligent gain control configuration, in order to configure the CFIA of the readout circuit into intelligent gain mode, it is necessary to set the fixed value gain control resistor R g Replaced with intelligent gain control resistors solved and configured based on the L-BFGS-B algorithm R g,tot ( T a ). Using commercial MF52 resistors, connect one 1 kΩ, one 5 kΩ, and one 4 kΩ in parallel to obtain a resistance of 689.66 Ω. R g0 , and then use a precision potentiometer to achieve the above resistance in parallel R gp (The resistance is precisely adjusted to 720 Ω), and then connected in series R gs (resistance is precisely adjusted to 1656 Ω), thus obtaining the R g,tot .

[0051] After configuring the intelligent gain control mode, experimental tests were conducted under ambient temperature changes of 0 ~ 50 ℃ and flow rates of 0 ~ 10 m / s. The experimental results are as follows Figure 14 As shown. Due to the intelligent gain control resistor R g,tot With the introduction of the gain adjustment, the negative temperature drift of the output is compensated and reduced to ±1.3% at 10 m / s.

[0052] The present invention discloses a flow sensing structure and a temperature drift compensation circuit for the flow sensing structure, wherein the temperature drift compensation circuit includes a control circuit and a readout circuit; the control circuit includes a first resistor, a second resistor, an intelligent temperature-controlled resistor, an ambient temperature reference resistor, a microheater and a first operational amplifier; wherein the first resistor, the second resistor, the intelligent temperature-controlled resistor, the ambient temperature reference resistor and the microheater are sequentially arranged in series, the connection point between the first resistor and the intelligent temperature-controlled resistor is connected to the positive input terminal of the first operational amplifier, the connection point between the second resistor and the microheater is connected to the negative input terminal of the first operational amplifier, and the connection point between the first resistor and the second resistor is connected to the negative input terminal of the first operational amplifier. The microheater is connected to the output end of the first operational amplifier; the readout circuit includes a Wheatstone bridge, a second operational amplifier, a first configuration resistor, a gain control resistor, and a second configuration resistor; wherein the microheater transfers heat to the Wheatstone bridge via thermal coupling, the first end of the Wheatstone bridge is connected to the negative input end of the second operational amplifier, the second end of the Wheatstone bridge is connected to the positive input end of the second operational amplifier, the positive output end of the second operational amplifier is connected to the first end of the first configuration resistor, the negative output end of the second operational amplifier is connected to the second end of the second configuration resistor, and the gain control resistor is arranged in series between the first configuration resistor and the second configuration resistor. The embodiment of the present invention adopts intelligent temperature-controlled resistors for configuration, so that the temperature difference output of the control circuit is a variable temperature difference. This design can significantly suppress temperature drift, solve the influence of temperature drift on measurement results, and improve measurement accuracy and reliability.

[0053] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and such modifications or substitutions are intended to be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A temperature drift compensation circuit for a flow sensor structure, characterized in that: The temperature drift compensation circuit includes a control circuit and a readout circuit; The control circuit includes a first resistor, a second resistor, an intelligent temperature-controlled resistor, an ambient temperature reference resistor, a microheater, and a first operational amplifier; wherein the first resistor, the second resistor, the intelligent temperature-controlled resistor, the ambient temperature reference resistor, and the microheater are sequentially arranged in series, the connection point between the first resistor and the intelligent temperature-controlled resistor is connected to the positive input terminal of the first operational amplifier, the connection point between the second resistor and the microheater is connected to the negative input terminal of the first operational amplifier, and the connection point between the first resistor and the second resistor is connected to the output terminal of the first operational amplifier; The readout circuit includes a Wheatstone bridge, a second operational amplifier, a first configuration resistor, a gain control resistor and a second configuration resistor; wherein the microheater transfers heat with the Wheatstone bridge through thermal coupling, the first end of the Wheatstone bridge is connected to the negative input end of the second operational amplifier, the second end of the Wheatstone bridge is connected to the positive input end of the second operational amplifier, the positive output end of the second operational amplifier is connected to the first end of the first configuration resistor, the negative output end of the second operational amplifier is connected to the second end of the second configuration resistor, and the gain control resistor is arranged in series between the first configuration resistor and the second configuration resistor.

2. The temperature drift compensation circuit for a flow sensor structure according to claim 1, characterized in that: The Wheatstone bridge comprises four bridge arms connected end to end, and each bridge arm is provided with a temperature sensing element.

3. The temperature drift compensation circuit for a flow sensor structure according to claim 1, characterized in that: The intelligent temperature-controlled resistor is a resistor with a positive temperature coefficient.

4. The temperature drift compensation circuit for a flow sensor structure according to claim 1, characterized in that: The readout circuit includes a first fixed-value resistor and a second fixed-value resistor. The first fixed-value resistor is arranged in parallel with the gain control resistor, and the second fixed-value resistor is arranged in series between the gain control resistor and the second configuration resistor.

5. The temperature drift compensation circuit for a flow sensor structure according to claim 1, characterized in that: The gain control resistor is a resistor with a negative temperature coefficient.

6. The temperature drift compensation circuit for a flow sensor structure according to claim 1, characterized in that: The micro heater has a serpentine structure.

7. A flow sensing structure, characterized in that: The flow sensing structure includes a substrate and the temperature drift compensation circuit according to any one of claims 2 to 6, wherein the temperature drift compensation circuit is arranged on the substrate, and the substrate is a silicon substrate.

8. The flow sensing structure according to claim 7, characterized in that: A cavity is provided on the substrate, the four temperature sensing elements and the micro heater are all provided on the cavity, and the four temperature sensing elements are symmetrically provided on both sides of the micro heater.

9. The flow sensing structure according to claim 7, characterized in that: The micro heater is arranged on a support member, both ends of the support member are connected to support beams respectively, and a wire electrically connected to the micro heater is arranged on the upper end surface of one of the support beams.

10. The flow sensing structure according to claim 7, characterized in that: The flow sensing structure is monolithically integrated using a standard CMOS-MEMS process.

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