A temperature compensation method for hot film flowmeter

Abstract

The present invention relates to a temperature compensation method for a thermal film flowmeter, wherein the thermal film flowmeter comprises a silicon plate and a heating thermistor and a temperature measuring thermistor arranged on the fluid side surface of the silicon plate, wherein solder joints are arranged on the silicon plate, wherein the heating thermistor is connected to the corresponding solder joints via leads, wherein the leads are arranged on the fluid side surface of the silicon plate. The method comprises: S1 constructing a Wheatstone bridge, S2 designing a resistor, and S3 obtaining a voltage value U3 across the third fixed resistor when measuring the flow rate of the fluid to be detected, and compensating the output voltage U′ by U′=M·U3, wherein #imgabs0# wherein T f is the temperature measured by the temperature measuring thermistor, thereby achieving temperature compensation.

Description

Technical Field

[0001] The invention relates to a temperature compensation method for a hot film flow meter. Background Art

[0002] There is a growing demand for micro flow sensors for industrial, automotive, household and medical applications. High sensitivity, fast response and low power consumption can be achieved using micromechanical (MEMS) processing. Considering that flow measurement is one of the most important issues for sensors, two principles seem to be the most promising among the possible implementations: differential pressure detection and thermal flow measurement. While differential pressure detection is very suitable for high-throughput liquid flow measurement, it is less suitable for low-flow liquid and gas flows. In this case, thermal measurement is superior.

[0003] Thermal flowmeters are basically heat-insulated structures with heaters and thermometers. Hot-wire and Hot-film flowmeters are a direct measurement method of thermal flowmeters. Other methods include calorimetric and time-of-flight. It can accurately measure flow and has a short response time.

[0004] Thermal film flow meters are based on the principle of convective heat transfer, that is, the heated object is cooled by the fluid passing through it, and the power dissipated by local cooling depends on the flow rate. A typical structure, for example, is a flow sensing element made of platinum or nickel thermistor film resistors on the surface of silicon, which can be used as the inner wall of the fluid channel after surface passivation. When the sensor is working, energy must be provided to keep the thermistor at a specific temperature higher than the surrounding fluid. When the fluid flows through the channel, heat energy is carried away. The faster the fluid flows, the more energy must be provided to keep the thermistor at a specific temperature, so the volume flow rate can be measured by the calibrated power consumption. The working principle of the thermal film flow meter can be simply described according to the following equation:

[0005]

[0006] P h is the heating power of the heating resistor; U h is the output voltage across the heating resistor; Rh is the heating resistor; h(v) is the heat transfer coefficient related to the flow velocity v, and the typical expression is h(v) = A + B·v n , A, B, n are all constants; T h is the temperature of the heating resistor; T f is the fluid temperature; S is the heating area, which is a constant.

[0007] In the case of hot film type, there are two ways to establish a one-to-one correspondence between flow velocity v and output:

[0008] (1) Constant heating power P h , then the velocity v is the temperature difference ΔT=(T h -T f ) is usually called the CP (Constant Power) model;

[0009] (2) For a constant temperature difference ΔT, the flow rate v is the power P h The functional relationship is usually called the CTA (Constant Temperature Anemometer) mode.

[0010] Since the heating resistor is a thermistor, it is difficult to realize the circuit of the CP mode to keep the heating power of the thermistor constant, while the CTA mode can be realized by a very simple negative feedback circuit.

[0011] However, changes in fluid temperature can cause output signal drift, resulting in inaccurate measurements.

[0012] Ref.1: Nam T, Kim S, Park S. The temperature compensation of a thermalflow sensor by changing the slope and the ratio of resistances[J]. Sensors&Actuators A Physical, 2004, 114(2-3): 212-218.

[0013] Ref.1 above introduces two typical temperature compensation methods, namely software compensation method and hardware compensation method. According to the comparison of the two temperature compensation methods in Ref.1, it can be seen that the software compensation method has low accuracy and complex circuit. However, a hardware compensation method called resistance ratio method is a cost-effective method because of its simple implementation and high accuracy. Its schematic diagram is as follows Figure 1 shown.

[0014] Figure 1 In the figure, Rh is a heating thermistor, Rf is a temperature measuring thermistor, where Rh and Rf are made of the same material, and R1, R2 and R3 are external fixed value resistors. Figure 1The resistance of one arm of the bridge, Rf, is much greater than that of Rh. The four resistors are configured as an unbalanced bridge at room temperature. If a voltage source Vde is applied, a voltage difference is generated between node1 and node2. The resulting current is fed back to the power supply through the amplifier to generate a gain current. Since the resistance of one arm of Rf is much greater than that of one arm of Rh, the current flowing through one arm of Rh is much greater than that flowing through one arm of Rf. Therefore, Rh is easily heated by the gain current, but Rf is hardly heated. The increase or decrease of Rh forces the bridge to balance, and the entire circuit reaches a stable working state. If the flow rate increases, forcing Rh to cool down, the resistance of Rh changes, breaking the balance of the bridge, then a gain current will be generated to heat Rh, causing the bridge to rebalance. If the fluid temperature T f , it is obvious that the temperature difference ΔT does not change; if the fluid temperature changes, the resistance value of Rf changes, and the balance of the bridge is broken. The heating of the gain current or the cooling of the fluid will rebalance the bridge. Since the initial temperatures of Rh and Rf are different during operation, if the temperature difference between Rf and Rh is to be constant, the resistance difference between Rh and Rf caused by the change is compensated by the resistor R1, R1 = a·Rh0·α·ΔT, where a = the resistance ratio of R2 / R3, called the overheating ratio, Rh0 is the resistance of Rh at the reference temperature T0, α is the temperature coefficient, the derivation process is shown in Ref.1, which will not be repeated here, and the specific use examples will be given in the implementation of the specification of the present invention. Therefore, the bridge always maintains balance during operation regardless of the change in flow rate or fluid temperature, and the temperature difference between Rh and Rf remains unchanged, that is, ΔT remains unchanged, which is a constant temperature difference.

[0015] In the above-mentioned prior art, the theoretical framework of the proportional resistance method is relatively clear, but in actual use, it is still found that slight changes in fluid temperature can cause output signal drift. Summary of the invention

[0016] The purpose of the present invention is to provide a temperature compensation method for a hot film flowmeter, which solves the problem that the change of fluid temperature may cause the output signal drift.

[0017] The present invention finds that the lead resistance R of the heating thermistor Rh L (See Figure 2 ) is affected by the fluid temperature T f The influence of lead resistance R L The existence of will cause the output signal to drift. The present invention aims to compensate for the lead resistance R L Affected by temperature T f The temperature difference ΔT between Rh and Rf caused by the influence drifts with the ambient temperature, making the flow measurement more accurate.

[0018] Here we discuss some of the constraints that underlie the compensation of lead resistance under fluid temperature T f Importance:

[0019] According to the lead resistance R L =ρl / (w·h), ρ is the resistivity, l is the lead length, w is the lead width, and h is the lead thickness. When the thickness h of the thin film resistor is consistent, if you want to reduce the resistance, you need to reduce the lead length l or increase the lead width w.

[0020] (1) MEMS mass production expects the total chip size to be small to reduce costs, which leaves limited space for the lead to expand in length and width to reduce resistance, that is, l and w tend not to be lengthened;

[0021] (2) The heating resistor Rh area is not expected to be affected by the packaging method. Since the wire bonding area often needs to be protected by glue, the protrusion of glue on the structure will interfere with the flow of the fluid in the accessories. In order to avoid the interference factors caused by the package, the solder joint area needs to be as far away from the Rh resistor as possible, that is, to ensure performance, l is difficult to shorten;

[0022] (3) Combining (1) and (2), under the premise of performance priority, l tends to be extended and w tends to be shortened. In many cases, it is difficult to make the resistance small enough to be ignored, and the lead layout needs to be carefully designed;

[0023] (4) The lead resistance can be effectively reduced by making the lead area resistor thicker or making a thin film structure in the lead area. However, since metal film deposition is often carried out by depositing the entire surface and then removing excess material, whether it is thick or a composite structure, a certain thickness of excess resistance will be removed from the surface of the Rh thin film resistor area. This not only increases the process steps and costs, but also introduces some uncertain factors such as thinning errors, adhesion layer and barrier layer processes;

[0024] (5) Do not worry about the lead resistance of the Rf resistor (the lead is Figure 3 The resistor and lead parts of Rf are immersed in the fluid and can be regarded as a series temperature sensor for measuring water temperature.

[0025] In summary, the lead resistance R of the heating thermistor Rh L The fluid temperature T f The influence is difficult to reduce and eliminate from the design and process, which will cause the temperature difference ΔT between Rh and Rf to drift with the ambient temperature.

[0026] In addition, although the flow rate v and power P h A one-to-one relationship is established, but For hot film flowmeter, Rh=Rh0·(1+α·(T f +ΔT-T0)). Therefore, the output voltage U h Or output current I h Still affected by the fluid temperature Tf impact.

[0027] In order to solve the guide wire resistance R L Affected by temperature T f The temperature difference ΔT between Rh and Rf caused by the ambient temperature drifts, and the output voltage U h Or output current I h and fluid temperature T f In some scenarios with a wide temperature range, a better temperature compensation method is necessary.

[0028] In order to achieve better temperature compensation, the technical solution adopted by the present invention is:

[0029] The present invention provides a temperature compensation method for a thermal film flowmeter, wherein the thermal film flowmeter comprises a silicon plate and a heating thermistor and a temperature measuring thermistor arranged on the fluid side surface of the silicon plate, wherein solder joints are arranged on the silicon plate, wherein the heating thermistor is connected to the corresponding solder joints via leads, and wherein the leads are arranged on the fluid side surface of the silicon plate;

[0030] It includes:

[0031] S1, constructing a Wheatstone bridge, wherein the first arm of the Wheatstone bridge is a heating thermistor and the lead connected in series, the second arm is a temperature measuring thermistor and the first fixed resistor connected in series, the third arm is the second fixed resistor, and the fourth arm is the third fixed resistor, the first arm, the second arm, the third arm and the fourth arm are connected end to end in sequence, a power line is connected between the first arm and the second arm, the third arm and the fourth arm are grounded, and an amplifier is also provided, one pole of the amplifier is connected between the second arm and the third arm, the other pole of the amplifier is connected between the fourth arm and the first arm, and the output end of the amplifier is connected to the power line;

[0032] S2, design resistance, determine reference temperature T0, overheat ratio a, expected temperature difference ΔT, Rh0, R L0 , Rh0 is the resistance value of the heating thermistor at T0, R L0 The resistance value of the lead at T0 is measured, the temperature coefficient α and the resistivity ρ0 of the heating thermistor at the reference temperature T0 are measured, the resistance value of the first fixed resistor is determined to be R1=a·Rh0·α·ΔT, the resistance value of the second fixed resistor is determined to be R2 and the resistance value of the third fixed resistor is determined by R2 / R3=a, and the resistance value of the third fixed resistor is determined by Rf0=a·(Rh0+R L0 ) Determine the resistance value Rf0 of the temperature measuring thermistor when the reference temperature is T0;

[0033] S3, when measuring the flow rate of the fluid to be detected, the voltage value across the third fixed resistor is obtained as U3, and the output voltage U' is compensated as U'=M·U3, where Where T f is the temperature measured by the temperature measuring thermistor.

[0034] Preferably, it also includes The flow velocity v of the fluid to be detected is determined, wherein U0, k, and n are determined by fitting the experimental data.

[0035] Preferably, the correction factor T in the formula f According to Rh+R L The voltage U on both sides hL Calculated, T f =U hL / (I h ·α·(1+b)·Rh0)-(1-α·T0) / α-ΔT / (1+b), where I h =U3 / R3, b=R L0 / Rh0.

[0036] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0037] The temperature compensation method of the hot film flowmeter of the present invention introduces a correction factor The compensated output voltage U′=M·U3 is calculated using the correction coefficient M, thereby achieving compensation for the output signal drift caused by the fluid temperature change. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Hereinafter, some specific embodiments of the present invention will be described in detail in an exemplary and non-limiting manner with reference to the accompanying drawings. The same reference numerals in the accompanying drawings indicate the same or similar components or parts. It should be understood by those skilled in the art that these drawings are not necessarily drawn to scale. In the accompanying drawings:

[0039] Figure 1 It is a schematic diagram of the temperature compensation principle of the constant temperature difference circuit using the resistance ratio method;

[0040] Figure 2 It is a schematic diagram of the chip structure layout;

[0041] Figure 3 is a schematic diagram of a temperature compensation circuit;

[0042] Figure 4 is the effect of the ratios b and c on the temperature difference ΔT;

[0043] Figure 5 is the temperature of different fluids T fSchematic diagram of the relationship between output voltage U and flow rate. DETAILED DESCRIPTION

[0044] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0045] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0046] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0047] The temperature compensation circuit provided by the present invention is as follows Figure 3 Similar to the circuit in Ref.1, R1, R2 and R3 are fixed value resistors, Rh is the heating thermistor, RL is the lead resistance of the heating thermistor on the chip, and Rf is the temperature measuring thermistor (including the lead resistance of the temperature measuring resistor).

[0048] The improved ratio resistor temperature compensation method is closely related to the resistor design on the chip and is carried out in the following steps:

[0049] 1. Test the flow sheet and measure the temperature coefficient α and resistivity ρ0 of thermistor (resistance changes linearly with temperature) (heating thermistor, temperature measuring thermistor, lead resistor are all thermistors and made of the same material) at the reference temperature T0. The resistance value of the thermistor within the allowable temperature range can be expressed as R=R0·(1+α·(T f -T0)), the resistivity can be expressed as ρ=ρ0·(1+α·(T f -T0)), R0 is the resistance at reference temperature T0;

[0050] 2. Select the design input parameters, including the reference temperature T0, superheat ratio a, expected temperature difference ΔT, Rh0, R L0 , where Rh0 is the resistance value of Rh at T0, R L0is the resistance value of the lead at T0, where a = the resistance ratio of R2 / R3, called the overheating ratio a, and the resistance value of the heating thermistor Rh = Rh0·(1+α·(T f -T0);

[0051] 3. Design Rh and R using numerical analysis methods such as finite element analysis L The structure and dimensions of the insulation structure ensure that the temperature on Rh is sufficiently uniform and that Rh and R L If the temperature gradient at the connection is large enough, the uncertainty resistance value caused by the temperature gradient is small enough to satisfy the Rh resistance when the bridge is balanced: Rh = Rh0·(1+α·(T f +ΔT-T0)) Assume that, similarly, R L =R L0 ·(1+α·(T f -T0)), R L0 For R L The resistance at T0;

[0052] 4. Calculate the fixed resistance R1 = a·Rh0·α·ΔT, select the fixed resistance R2 or R3, and then select another resistance according to R2 / R3=a to ensure that the selected resistance can meet the standard resistance. If it meets the standard resistance, proceed to the next step. If not, return to step 2.

[0053] 5. Calculate the temperature measuring resistance Rf0 = a (Rh0 + R L0 ), Rf0 is the resistance of Rf at T0. The geometric dimensions of Rf on the chip are designed according to the resistance value of Rf0, and the temperature field is calculated using numerical analysis methods such as finite element analysis to ensure that Rf is far enough from Rh to satisfy Rf=Rf0·(1+α·(T f -T0);

[0054] 6. The above 5 steps can ensure that the temperature difference ΔT between Rf and Rh is approximately constant, and R L The temperature effect of the resistance has been compensated by hardware. L It is inseparable on the chip, so the output voltage U can be read at Rh+R L Both sides, R3 both sides or Rh+R L +R3 sides, for simplicity, here the output voltage U is selected on both sides of R3, that is, U = U3. Output voltage U and fluid temperature T f The correlation needs to be compensated by software, and the correction coefficient is introduced here.

[0055] 7. Calculate T f , T f There are many ways to obtain , here we can use Rh+R L The voltage U on both sideshL Calculate, then T f =U hL / (I h ·α·(1+b)·Rh0)-(1-α·T0) / α-ΔT / (1+b), I h =U3 / R3, b=R L0 / Rh0;

[0056] 8. Calculate the output voltage after compensation U0 and k are constants based on fluid characteristics, v is the flow rate of the fluid to be tested, which is the physical quantity to be measured, and U0, k, and n are determined by fitting experimental data;

[0057] 9. Complete temperature compensation.

[0058] An example is given below.

[0059] Assuming an ideal Pt thermistor material, the reference temperature T0 is 0°C, Rh0 is 100Ω, R L0 is 10Ω, the temperature coefficient α is 3850ppm / ℃, the overheat ratio a is 20, so R1 is 77Ω and Rf0 is 2200Ω. If the output power on both sides of Rh is 20mW, R3 is 100Ω, calculate the following conditions:

[0060] (1) Assume that the fluid temperature is T f is 20℃, and the temperature difference ΔT is 10℃.

[0061] When the fluid temperature is 20℃ and the bridge is balanced, Rh is 111.55Ω at 30℃, R L At 20℃, it is 10.77Ω, and Rf is 2369.4Ω at 20℃. According to the bridge balance condition, it can be deduced that a′=(Rf+R1) / (Rh+R L )=20, obviously a′=a, which is established. Then the actual output voltages on both sides of R3 are U3 is 1.339V, and U′ is 1.4142V.

[0062] (2) Assume that the fluid temperature is T f is 50℃ and the temperature difference ΔT is 10℃.

[0063] When the fluid temperature is 50℃ and the bridge is balanced, Rh is 123.1Ω at 60℃, R L At 50℃, it is 11.925Ω, and Rf is 2623.5Ω at 50℃. According to the bridge balance condition, it can be deduced that a′=(Rf+R1) / (Rh+R L )=20, obviously a′=a, which is established. Then the actual output voltages on both sides of R3 are U3 is 1.2746V, and U′ is 1.4142V.

[0064] (3) Assume that the fluid temperature is T f is 80℃, the temperature difference ΔT is 10℃

[0065] When the fluid temperature is 80℃ and the bridge is balanced, Rh is 134.65Ω at 90℃, R L At 80℃, it is 13.08Ω, and Rf is 2877.6Ω at 80℃. According to the bridge balance condition, it can be deduced that a′=(Rf+R1) / (Rh+R L )=20, obviously a′=a, which is established. Then the actual output voltages on both sides of Rh are U3 is 1.2187V, and U′ is 1.4142V.

[0066] According to the above calculation, the temperature difference ΔT remains unchanged, Rh, Rf and R L The resistance changes with temperature T f The bridge balance condition is always satisfied, so the temperature compensation of constant temperature difference is established. The output voltage U′ after compensation does not change with T f changes, so the temperature compensation of the output voltage U3 is established.

[0067] The beneficial effects of the present invention are:

[0068] 1. Can effectively reduce the lead resistance Rh L With the fluid temperature T f Changes cause a drift in the temperature difference ΔT.

[0069] If according to Figure 1 The ratio resistance method shown Figure 2 The problem shown in the figure, the temperature difference ΔT is affected by b and the result is as follows Figure 4 As shown. It can be seen that even if the lead resistance R L A ratio b of only 1% of the heating thermistor Rh will also have a significant impact on the temperature difference ΔT. Not only does ΔT deviate from the expected 10°C, but the impact becomes greater and the temperature drift becomes more severe as the ratio b increases.

[0070] The improved temperature compensation method proposed in the invention can greatly suppress the drift of the temperature difference ΔT. According to step 5, Rf0 = a·(Rh0 + R L0 ), then R L The temperature compensation resistor R at the reference temperature T0 LC0 =a·R L0 , R LC0 There is a certain relative error c, c = (R LC0 -R LC0 ′) / R LC0 , R LC0 ′ is the actual temperature compensation resistance. Figure 4 As shown, the effect obtained when the relative error c reaches 10% is consistent with the effect when the ratio c reaches 1%, and the drift of ΔT is reduced by 10 times.

[0071] 2. It can suppress the output voltage U from changing with the fluid temperature T f Drift.

[0072] like Figure 5 As shown, the output voltage U changes with T f The drift can be corrected by the method of steps 5-8 to obtain the drift that does not change with T f The curve after compensation of the change.

[0073] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the contents of the present invention and implement them accordingly. They cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A temperature compensation method for a thermal film flowmeter, the thermal film flowmeter comprising a silicon plate and a heating thermistor and a temperature measuring thermistor arranged on the fluid side surface of the silicon plate, the silicon plate being provided with solder joints, the heating thermistor being connected to the corresponding solder joints through leads, the leads being arranged on the fluid side surface of the silicon plate; It includes: S1, constructing a Wheatstone bridge, wherein the first arm of the Wheatstone bridge is a heating thermistor and the lead connected in series, the second arm is a temperature measuring thermistor and the first fixed resistor connected in series, the third arm is the second fixed resistor, and the fourth arm is the third fixed resistor, the first arm, the second arm, the third arm and the fourth arm are connected end to end in sequence, a power line is connected between the first arm and the second arm, the third arm and the fourth arm are grounded, and an amplifier is also provided, one pole of the amplifier is connected between the second arm and the third arm, the other pole of the amplifier is connected between the fourth arm and the first arm, and the output end of the amplifier is connected to the power line; S2, design resistance, determine reference temperature T0, overheat ratio a, expected temperature difference ΔT, Rh0, R L0 , Rh0 is the resistance value of the heating thermistor at T0, R L0 The resistance value of the lead at T0 is measured, the temperature coefficient α and the resistivity ρ0 of the heating thermistor at the reference temperature T0 are measured, the resistance value of the first fixed resistor is determined to be R1=a·Rh0·α·ΔT, the resistance value of the second fixed resistor is determined to be R2 and the resistance value of the third fixed resistor is determined by R2 / R3=a, and the resistance value of the third fixed resistor is determined by Rf0=a·(Rh0+R L0 ) Determine the resistance value Rf0 of the temperature measuring thermistor when the reference temperature is T0; S3, when measuring the flow rate of the fluid to be detected, the voltage value across the third fixed resistor is obtained as U3, and the compensated output voltage U′=M·U3 is calculated, where , where T f is the temperature measured by the temperature measuring thermistor.

2. The temperature compensation method of hot film flowmeter according to claim 1, characterized in that: It also includes The flow velocity v of the fluid to be detected is determined, wherein U0, k, and n are determined by fitting the experimental data.

3. The temperature compensation method for a hot film flowmeter according to claim 1, characterized in that: Correction factor T in the formula f According to Rh+R L The voltage U on both sides hL Calculated, T f =U hL / (I h ·α·(1+b)·Rh0)-(1-α·T0) / α-ΔT / (1+b), where I h =U3 / R3, b=R L0 / Rh0.

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